Two-stroke cycle engine

ABSTRACT

A two-stroke cycle engine using a preceding air-layer for scavenging and having a scavenger passage connected to a branching scavenger passage opened to a scavenging port and a connecting passage linking the air and fuel passages so that negative pressure in the air passage forces the fuel-air mixture in the fuel passage into the air passage. The engine also has a removable guide with a surface forming a curved smooth channel which is attachable to the scavenger passage in the crankcase from the mounting surface, and forms a portion of the scavenger passage with the curved channel. The blow-up angle of the scavenger passage varies along the circumference of the cylinder, and the crankcase is configured such that the front and rear portions are separated by a block, and a scavenger passage is provided inside both said front and rear portions of the crankcase and cylinder. Two parallel air passages extend from the air cleaner, the first connected to the air passage, and the second connected to the air inlet of the carburetor to provide air for the fuel passage, and a choke valve on the air cleaner is provided to open and close the first and second air passages.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of co-pending application Ser. No.10/634,918, filed Aug. 6, 2003, now U.S. Pat. No. ______, which in turnwas a division of application Ser. No. 10/338,727, filed Jan. 9, 2003,now U.S. Pat. No. 6,729,276, which was a division of application Ser.No. 10/152,035, filed May 22, 2002, now U.S. Pat. No. 6,564,761, whichwas a division of application Ser. No. 09/832,802, filed Apr. 12, 2001,now U.S. Pat. No. 6,4008,805, which was a division of application Ser.No. 09/558,596, filed Apr. 26, 2000, now U.S. Pat. No. 6,257,179.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns a two-stroke cycle engine which uses a layer ofscavenging air pressurizing the crankcase. More specifically, itconcerns a small two-stroke cycle engine using a preceding air-layer forscavenging, which drives a layer of scavenging air in advance of thefuel-air mixture.

2. Description of the Related Art

Two-stroke cycle engines belonging to the prior art take advantage ofthe fact that a negative pressure is created in the crankcase when thepiston reaches the top of its stroke. This negative pressure causes thefuel-air mixture to be sucked into the crankcase. When the pistonreaches the bottom of its stroke, the pressurized fuel-air mixture inthe crankcase reaches the scavenging port and is conducted from thecrankcase into the combustion chamber. The fuel-air mixture fills thecombustion chamber, pushing the exhaust gases ahead of it. In thisscavenging process, the opening duration of the scavenging port and theexhaust port experiences significant overlap, with the result thatapproximately 30% of the fuel-air mixture is sucked out with the exhaustgases. This is the primary cause of the large component of THC (totalhydrocarbons) in the exhaust, and it results in the wastage of fuel.

To reduce the quantity of fuel-air mixture which is pushed out of thecombustion chamber, scavenging air designs which drive a layer of airahead of the fuel-air mixture have been proposed. In engines which usescavenging air, the fuel-air mixture goes into the crankcase as thepiston travels upward in the intake process. At the same time, air issucked into the crankcase through a scavenger passage connected to thescavenging port so that the passage is filled with air. In thecombustion and exhaust processes which occur when the piston drops andthe scavenging port is open, the air in the scavenger passage is forcedinto the combustion chamber ahead of the fuel-air mixture to scavengethe exhaust gases from the combustion. Immediately after the scavengingair, the fuel-air mixture is admitted into the combustion chamber. Thisscavenging-air method reduces the quantity of fuel-air mixture which ispushed out of the combustion chamber to one third that which occurredwith prior art engines.

A design for a scavenging-air two-stroke cycle engine which forces alayer of air ahead of the fuel-air mixture, in which the fuel and airvalves on the carburetor are realized as a single valve, is disclosed inthe Japanese Patent Publication (Kokai) 10-252565.

In the prior art scavenging-air engine which drives a layer of air aheadof the fuel-air mixture, the preceding layer of air admitted to thecylinder and crankcase through the air control valve was routed throughthe same number of passages (either two or three) as there werescavenging ports downstream from the air control valve. These wereconnected to the passages for the scavenging ports of the cylinders byrubber tubes. The air was fed through lead valves on the scavengerpassages to passages on the cylinder and crankcase.

The air introduced via the air control valve was sucked into thecrankcase temporarily when the cylinder of the piston was pressurized.When the piston dropped and scavenging occurred, the scavenging air wasled into the combustion chamber from the scavenging port.

In another prior art design proposed in the Japanese Patent Publication(Kokai) 7-139358, an air passage was provided which fed into thescavenger passage at a location adjacent to the scavenging port. Anon-return valve was provided on the air passage, as was a controlvalve. The control valve was interlinked with the operation of theengine throttle. In this engine, the crankcase experienced negativepressure when the piston was up. At the same time that the fuel-airmixture was sucked into the crankcase through its supply port, thenon-return valve was opened and the air was sucked in through the airpassage. This air would completely or partially fill the scavengerpassage. When the piston fell during the ignition and exhaust processesand the scavenging port was opened, first the air would rush into thecombustion chamber and then the fuel-air mixture would be supplied.

With this prior art technique, a means was devised which would supplythe air from the scavenging port to scavenge the combustion chamberquickly at the start of the scavenging process so as to minimize thequantity of fuel-air mixture lost through the exhaust port. This deviceadmitted the fuel-air mixture from the crankcase into the combustionchamber via the scavenging port with a slight delay after the scavengingair was admitted.

This sort of two-stroke cycle engine which admitted a layer of air infront of the fuel-air mixture reduced the quantity of mixture exhaustedwith the combustion gases, prevented an excessive quantity of THC (totalhydrocarbons) from being exhausted, and minimized the quantity of fuelwasted.

In the preceding air-layer type two-stroke cycle engine proposed in theJapanese Patent Publication (Kokai) 10-252565, the preceding air wasbrought in through a number of rubber tubes with lead valves which wasequal to the number of scavenging ports. The design thus required alarge number of parts and assembly processes, both of which drove thecost up. Furthermore, the supply passages for the air were provided onthe outside of the cylinder, so the dimensions of the engine in itsaxial direction were increased.

In a two-stroke cycle engine, combustion must be kept stable bysupplying a rich mixture with little air when the engine is operatingunder a light load, including when it is idling, and a comparativelythin mixture when it is operating under a heavy load. This will reducefuel consumption and decrease the harmful component of the exhaust gas.However, in the prior art design proposed in the Japanese PatentPublication (Kokai) 7-139358, the airflow supplied via the supplypassages during scavenging is not controlled to conform to the operatingstate of the engine.

Under light load conditions, then, such as when the engine is idling,too much air is supplied; and it would be difficult to stabilizecombustion by limiting the quantity of air admitted to produce a richmixture. Similarly, it would be difficult to maintain a thin mixtureunder heavy load conditions in order to reduce the pollutants in theexhaust gas and lower the fuel consumption.

In the invention disclosed in the Japanese Patent Publication (Kokai)9-125966, a mixture control valve is provided to open and close themixture passage which connects the carburetor to the crankcase, and anair control valve is provided to open and close the air passage whichconnects the air cleaner. The mixture control valve and air controlvalve are linked so that it is possible to control the flow rate of thefuel-air mixture and that of the air in such a way that their ratioremains constant.

In this type of preceding air-layer type two-stroke cycle engine, whenthe engine is idling the negative pressure in the air passage increasesuntil it is higher than that in the fuel mixture passage. This causesthe throttle to open more, suddenly increasing the speed of the engine.The delay in the fuel supply allows the excessively rich fuel mixture tobe thinned out by radically increasing the quantity of preceding air.The extra air reduces the concentration of the fuel mixture.

But when the engine is operating at high speed, an increase in thequantity of air will not be followed by an increased quantity of fuel.The concentration of the fuel will decrease and proper combustion willno longer be possible. Problems with acceleration or engine cut-off mayresult.

However, in the inventions disclosed in the Japanese Patent Publications7-139358, 10-252565 and 9-125966, no means are provided to control theratio of air flow to air-fuel mixture flow during normal operation so asto prevent an excessive quantity of air from being supplied when theengine suddenly accelerates as was described above.

Furthermore, when this sort of two-stroke cycle engine is used in alawnmower, in many cases it must operate while mounted obliquely. Whenan obliquely mounted engine operates, fuel collects in the portion belowthe passage for the fuel-air mixture. When the position of the enginechanges, this fuel is sucked in suddenly, resulting in combustionproblems due to excess fuel in the engine.

However, the prior art techniques did not offer any method to counteractproblems arising from the engine being operated while mounted obliquely.

Generally in two-stroke cycle engines, the passage for the scavengingair which leads into the chamber inside the crankcase follows a smoothcurve inside the crankcase, goes through the surface where the crankcaseis attached to the cylinder, and is connected to the scavenging port inthe cylinder.

FIG. 39 shows an example of a scavenger passage in a two-stroke cycleengine belonging to the prior art. In this drawing, 1 is the engine,which is configured as follows.

2 is the cylinder; 5 is the crankcase. The cylinder 2 and crankcase 5are fixed to each other with gasket 3 between them by bolts 110 atsurfaces 04 and 05. 6 is the crankshaft, 7 is the cylinder head, 10 isthe air passage and 60 is the center of the crankshaft.

9 is the scavenging port, which opens into the side of the cylinder 2.109a is the scavenger passage formed in the cylinder 2, which connectsto the scavenging port 9. 109c is the inlet for the scavenger passageformed in the crankcase 5, which opens into the crank chamber. 109b isthe scavenger passage in the crankcase 5. It follows a smooth curveinside the crankcase 5 and connects scavenger passage 109a in thecylinder 2 with scavenging inlet 109c.

In this two-stroke cycle engine, the scavenger passage comprises passage109b in crankcase 5 and 109a in cylinder 2, which meet at the surfaces04 and 05. Because scavenger passage 109b in crankcase 5 is curved, ithas a portion 16 which protrudes between the upper wall of the passage109b and surface 04.

In the Japanese Patent Publications 58-5423 and 58-5424, scavengerpassages are disclosed such that a curved scavenger passage in thecrankcase and a scavenger passage in the cylinder come together at thesurface where the crankcase and the cylinder are fixed to each other.

However, in the configuration of the scavenger passage in the two-strokecycle engine shown in FIG. 39, scavenger passage 109b in crankcase 5 isa curved passage with a portion 16 which protrudes between the upperwall of the passage 109b and surface 04. When the crankcase is cast, thedie to form scavenger passage 109b in crankcase 5 cannot be removed as asingle die in the direction of axis 61 of the cylinder.

With the prior art design shown in FIG. 39, then, to enable the die forscavenger passage 109b to be removed, several different dies werecombined to cast the scavenger passage. This complicated the castingwork and increased the number of casting processes. Also, because itrequired combining a number of dies, the probability of a defective castdue to slippage of a die increased.

In the Japanese Patent Publications (Kokai) 58-5423 and 58-5424, too, aprotruding portion is formed between the upper wall of the scavengerpassage and the surface where the cylinder is fixed to the crankcase.These designs, then, suffer from the same problems as were justdescribed.

Further, with the prior art design shown in FIG. 39, the curvedscavenger passage in the crankcase and the scavenger passage in thecylinder meet to form a scavenger passage which goes through the surfacewhere the crankcase and the cylinder are fixed to each other. The fuelin the fuel-air mixture which flows through the scavenger passage willthus seep into the microscopic gap between surfaces 04 and 05 wheregasket 3 is inserted. When the engine is operated in an obliqueposition, this fuel will return to the scavenger passage and result indefective combustion.

In the Japanese Patent Publications (Kokai) 58-5423 and 58-5424, too, ascavenger passage in the crankcase and a scavenger passage in thecylinder meet to form a common passage which goes through the surfacewhere the crankcase and the cylinder are fixed together. These designsthus suffer from the same problem which is described above.

In a two-stroke cycle engine with a pressurized crankcase, the designtakes advantage of the fact that a negative pressure is created in thecrankcase when the piston reaches the top of its stroke. The fuel-airmixture is sucked into the crankcase through the air inlet. When thepiston reaches the bottom of its stroke, the scavenging port opens andthe pressurized fuel-air mixture in the crankcase is conducted from thecrankcase into the combustion chamber via the scavenger passage and thescavenging port. The fuel-air mixture fills the combustion chamber,pushing the exhaust gases ahead of it.

In the scavenging process in a two-stroke cycle engine, the openingduration of the scavenging port and the exhaust port experiencessignificant overlap. To address this problem, a number of devices havebeen proposed to prevent the fuel-air mixture from being sucked out withthe combustion gases and to insure that the mixture fills the combustionchamber uniformly.

One such device is proposed in the Japanese Utility Model Publication(Kokai) 1-44740. In this proposal, a two-stroke cycle engine has twoscavenger passages, one on the right and one on the left, which lead upfrom the crankcase. Their upper ends curve toward the axial direction ofthe cylinder, and they lead into the cylinder. The angles at which thesurfaces of the upper walls of the curving scavenger passages meet thecylinder vary continuously from one side to the other.

In this sort of scavenging two-stroke cycle engine with a pressurizedcrankcase, it is necessary to reduce the quantity of fuel-air mixturewhich escapes with the exhaust gases, eliminate the exhaust of a largequantity of THC (total hydrocarbons) and minimize the wastage of fuel.

In the scavenging two-stroke cycle engine with a pressurized crankcaseproposed in the Japanese Utility Model Publication (Kokai) 1-44740, theangles at which the surfaces of the upper walls of the tops of thescavenger passages meet the cylinder, that is, the angles at which theair is blown into the cylinder, do vary continuously from one side ofthe scavenging port to the other. However, as can be seen in FIG. 3 ofthe same publication, the angles θ of the airflow differ from eachother. The portion (θa) nearer the exhaust port, which is shown in FIG.3 (a) is larger than the portion (θc) nearer the intake port, which isshown in FIG. 3 (c).

Thus in the prior art design proposed in the Japanese Utility ModelPublication (Kokai) 1-44740, the angle θ of the airflow in the locationcloser to the exhaust port is made larger. As a result, after thefuel-air mixture forced into the combustion chamber from the portion ofthe scavenger passage closer to the exhaust port reaches the top of thecombustion chamber, it is liable to be caught in the flow of combustiongases travelling toward the exhaust port. The fuel-air mixture suppliedfrom the location closer to the exhaust port, then, is likely to escapeout the exhaust port with the combustion gases which comprise theexhaust gas. This will increase the quantity of the THC (totalhydrocarbons) which are exhausted and the quantity of fuel which iswasted. The scavenging efficiency will decrease, the density of thefuel-air mixture filling the combustion chamber will be lower, and theengine output will go down.

In a two-stroke cycle engine which is used in a lawnmower, the longscavenger passage which connects the scavenging port to the crankcaseand supplies the air-fuel mixture from the crankcase to the combustionchamber must be formed in both the crankcase and the cylinder. Thecrankcase and cylinder, which are generally made of cast aluminum, mustassume a complicated shape, so that their casting requires manyprocesses.

Two-stroke cycle engines for universal applications have been proposedin the Japanese Patent Publication (Kokai) 58-5424 and the JapaneseUtility Model Publication (Kokai) 4-26657, among others.

In the Japanese Patent Publication (Kokai) 58-5424, the crankcase hasboth a primary and an auxiliary scavenging port. The two scavengerpassages which lead into the interior of the crankcase, i.e., into thecrank chamber, go from the interior of the crankcase through the surfacewhere the crankcase and the cylinder are fixed together. In thecylinder, these two scavenger passages connect to the primary andauxiliary scavenging ports.

In the Japanese Utility Model Publication (Kokai) 4-26657, two pairs ofcurved scavenger passages go from the interior of the crankcase throughthe surface where the crankcase and the cylinder are fixed together andthen through the interior of the cylinder.

As has been discussed, this is an air-layer type scavenging two-strokecycle engine. In it, a long scavenger passage which connects thescavenging port and the crankcase is formed in the interior portions ofthe crankcase and the cylinder. In addition, an air passage is formed inthe cylinder to transport the preceding air to the scavenging port. Thisair passage connects to some intermediate location on the scavengerpassage. The crankcase and cylinder, which are generally made of castaluminum, must be shaped in such a way that the scavenger passage formsa smooth channel in order to minimize the resistance experienced by thefuel-air mixture and the airflow. The shapes of the dies for the castingmust be simple, the number of dies must be small, and the engine must beable to be produced in a small number of production processes.

In the design in the Japanese Patent Publication (Kokai) 58-5424,however, the inventors limited their improvement to giving the two longprimary and auxiliary scavenger passages in the crankcase and cylinder asmooth contour and so reducing the resistance of the two channels. Theydid not devote any attention to improving the shape or number of diesused when the crankcase and cylinder were cast or to improving thecasting work by reducing the number of casting processes required.

In the design proposed in the Japanese Utility Model Publication (Kokai)4-26657, the inventors limited their improvement to providing a shapefor a curved scavenger passage running from the interior of thecrankcase through the interior of the cylinder and preventing thefuel-air mixture from escaping toward the exhaust side. In this priorart design, then, just as with that discussed above, no considerationwas given to the casting of the crankcase and cylinder.

In a two-stroke cycle scavenging engine using a layer of air, there is ascavenger passage which connects the scavenging port on the side of thecylinder and the crankcase; an air passage which connects to thescavenger passage at a point midway along its length and suppliesscavenging air from the air cleaner to the scavenger passage; and apassage which supplies the fuel-air mixture produced in the carburetorto the crankcase. Before the fuel-air mixture is supplied from thescavenging port to the combustion chamber, a mass of preceding airfiltered by the air cleaner is conducted into the combustion chamber byway of the air passage, scavenger passage and scavenging port. This airscavenges the chamber, enhancing both the scavenging and the combustionefficiency.

Two inventions which have proposed such two-stroke cycle scavengingengines which use a layer of air are those disclosed in the JapanesePatent Publications (Kokai) 9-125966 and 10-252565.

In the invention disclosed in the Japanese Patent Publication (Kokai)9-125966, there are two air cleaners. The outlet of one of the aircleaners connects via the carburetor to the supply passage for thefuel-air mixture. The outlet of the other air cleaner connects through acontrol valve to the passage which supplies the preceding air to thescavenger passage.

In the invention disclosed in the Japanese Patent Publication (Kokai)10-252565, the passage which supplies the fuel-air mixture from thecarburetor to the crankcase is parallel to the passage which suppliesthe preceding air to the scavenger passage. The air inlet of thecarburetor and the inlet of the air supply passage are connecteddirectly to the outlet of the air cleaner.

When this air layer-type scavenging two-stroke cycle engine is startedup, the quantity of air which is to go through the air supply passagemust be controlled and a negative pressure must be obtained. Thequantity of fuel-air mixture supplied to the combustion chamber from theair mixture supply passage by way of the crankcase and scavenger passagemust be increased to produce a rich mixture and so improve the startingcharacteristics of the engine.

In the invention proposed in the Japanese Patent Publication (Kokai)9-125966, the air control valve is shut during start-up, so the airsupply passage is closed. The air passage on the carburetor is open, andthe fuel-air mixture supplied from the carburetor to the air mixturesupply passage is increased to produce a rich mixture. However, thisdesign requires two air cleaners, so the configuration is complicatedand large, which drives up the equipment cost.

In the invention proposed in the Japanese Patent Publication (Kokai)10-252565, the air inlet of the carburetor and the inlet of the airsupply passage are connected directly to the outlet of the air cleaner.For this reason it would be extremely difficult to completely shut offthe preceding air, that is, the air which flows into the air supplypassage, at the outlet stage of the air cleaner during start-up.

It is thus impossible, in this invention, to supply a rich mixture tothe combustion chamber. And since it is difficult to achieve a highnegative pressure as well, the start-up characteristics are unavoidablypoor.

Furthermore, in the two prior art designs discussed above, the chokevalve which closes off the air passage when the engine is started up isgenerally a rocking choke valve which rotates about its valve shaft.When the engine is connected to a lawnmower or other working machine andis to be operated, the operating lever of the choke valve gets in theway when the engine is started with the recoil starter. Also, therocking diameter of the operating lever is considerable, which makes itdifficult to operate and so affects the ease of operation.

SUMMARY OF THE INVENTION

The present invention was developed to address the problems associatedwith the prior art.

The first objective of this invention is to simplify the configurationof the scavenger passage in a scavenging two-stroke cycle engine using alayer of air, to make the engine smaller and lighter, to reduce thenumber of parts and processes required to produce the engine, and toreduce the production cost.

The second objective of this invention is to provide an air-layer-typescavenging two-stroke cycle engine such that the quantity of fuel-airmixture exhausted with the combustion gases is reduced, and at the sametime the quantity of air supplied to the combustion chamber through thescavenging port is controlled via a cut-off valve so that it remains inproper proportion to the quantity of fuel-air mixture. In such anengine, an appropriate concentration of fuel-air mixture is maintainedthrough the entire operating range of the engine. Combustion remainsconstant when the engine is operated under a light load, and the rate offuel consumption and the proportion of pollutants in the exhaust arereduced when the engine is operated under a heavy load.

The third objective of this invention is, in an air-layer-typescavenging two-stroke cycle engine, to prevent the concentration of thefuel mixture from becoming too thin due to an excessive intake of airwhen the engine accelerates suddenly and so maintain proper combustion,thus preventing the engine from hesitating or cutting out.

The fourth objective of this invention is to prevent fuel fromcollecting at the bottom of the fuel passage when the engine isinstalled obliquely, and so prevent the defective combustion which wouldresult from an excess of fuel.

The fifth objective of this invention is to make it possible to cast thescavenger passage for a two-stroke cycle engine in such a way that asingle die could be used and the piece could be removed from the die inthe axial direction of the cylinder. The casting process would besimplified and the number of sub-processes reduced. This would preventdefects resulting from the dies slipping. It would also prevent theimperfect combustion which occurs when the engine is mounted obliquelydue to fuel flowing back into the scavenger passage from the surfacewhere the cylinder and crankcase are fixed together.

The sixth objective of this invention is, in a two-stroke cycle engineusing pressurized air to scavenge the crankcase, to prevent the fuel-airmixture from escaping out the exhaust port, the reduce the quantity ofTHC (total hydrocarbons) in the exhaust, to improve the efficiency ofthe scavenging, to increase the concentration of the fuel-air mixture inthe combustion chamber, and to improve the combustion and so enhance theoutput of the engine.

The seventh objective of this invention is to provide a two-stroke cycleengine with a scavenger passage in the crankcase and cylinder and amethod of casting such an engine such that a smooth scavenger passagewith little flow resistance could be formed, simply-shaped dies could beused to cast the engine, the number of dies could be reduced, and thenumber of casting processes could be reduced.

The eighth objective of this invention is to provide a two-stroke cycleengine with a simple, compact, low-cost configuration which would beable to supply a rich fuel-air mixture to the combustion chamber duringstart-up, and which would enjoy improved start-up characteristics as aresult of achieving a high negative pressure.

The ninth objective of this invention is to provide a valve such as achoke valve with a simple and compact configuration to improve theoperating characteristics of the engine.

This invention, then, comprises an exhaust port on the side of thecylinder; a scavenging port on the side of the cylinder; a fuel passage,which supplies fuel-air mixture to the crank chamber through the intakeport on the side of the cylinder during the time of the elevation of thepiston; an air passage, which supplies scavenging air from the aircleaner towards the inner side of the engine; an insulator, in which thefuel passage and the air passage run in parallel; a non-return valve,which is provided on the insulator facing towards the inner side of theengine, to allow the scavenging air to flow only towards the inner sideof the engine; a pair of branching air passages to connect an air supplychamber provided at the inner side of the non-return valve and abranching scavenger passage opened to the scavenging port, which areprovided within the wall of the cylinder: and a pair of scavengerpassages, one end of which is connected to the scavenging port, andanother outlet end of which is opened to the crank chamber, the pair ofscavenger passages are provided within the wall of the crankcase.

With this invention, scavenger passages are formed in halves, with onehalf of each inside the walls of the crankcase and the other half insidethe walls of the cylinder. This results in long scavenger passageswhich, when filled with air, can scavenge the crankcase ahead of thefuel-air mixture. Since the mixture is supplied after the crankcase iscompletely scavenged, the quantity of mixture lost through the exhaustport can be minimized.

Since all the scavenging and air passages are formed inside thecrankcase and cylinder, no external pipes or mounting hardware areneeded to create air passages. Both the parts count and the number ofassembly processes are therefore reduced.

Ideally, as in claim 2, in addition to the configuration disclosed inclaim 1, the end surface of the outlet end of the scavenger passage inthe crankcase forms right angles with respect to the axis of thecrankshaft, and a microscopic gap is created between the end surface ofthe outlet and the end surfaces of the crank webs which areperpendicular to the crankshaft, which constitutes disk valves, as theopening area of the outlet of the scavenger passage varies as the crankwebs rotate.

As in claim 3, the opening area of the outlet of the scavenger passageis formed so that the opening area opens more with the rotation of thecrank webs as the opening area uncovered by the crank web grows larger.

If configured as described above, the area of the outlets of thescavenger passages will increase with the rotation of the crank webs. Inthis way the velocity of the scavenging air which flows from the outletsthrough the scavenger passages and from the scavenging ports into thecombustion chamber can be controlled. This allows us to reduce thequantity of fuel-air mixture which is dragged into the exhaust gasstream, and so minimize the loss of fuel-air mixture.

Ideally, as in claim 4, the branching air passages and the branchingscavenger passages formed on either side of the cylinder are surroundedby virtually parallel walls which run in the same direction.

If configured in this way, the walls of the branching scavenging and airpassages can be formed integrally to and virtually parallel with thecylinder. This allows the cylinder to be cast using a single slidingdie. This simplifies the configuration of the die and reduces the costof producing it.

The invention disclosed in claim 5 of this application comprises atwo-stroke cycle engine using a preceding air-layer for scavenging withan exhaust port on the side of the cylinder; a scavenging port on theside of the cylinder; an intake port on the side of the cylinder fuelpassage, which supplies fuel-air mixture through a mixture control valveon the carburetor to the crank chamber during the time of the elevationof the piston; a scavenger passage opened to the scavenging port; an airsupply port, which supplies scavenging air from the air cleaner to thescavenger passage; a cam which is interlocked with the mixture controlvalve; a cam follower which engages with the cam; and an air controlvalve in the upstream of an air passage which controls the diameter ofthe air passage, and the air control valve being operated by the cam andthe cam follower in such a way as to supply a quantity of scavenging airproportional to the quantity of fuel-air mixture determined by theopening of the cam and the mixture control valve to control the fuel-airmixture.

Ideally, as in claim 6, in addition to the configuration disclosed inclaim 5, the air control valve comprises a valve seat midway along theair passage and an umbrella-type valve which can be attached to orremoved from the valve seat and which opens and closes the air passage,the cam is fixed to the rotary shaft of the mixture control valve, thecam is configured with an inner cam which is formed on the inside of theedge at a given height raised up on the outer side along thecircumference so that, if a spring exerts force in the direction whichcloses the air control valve, when the edge of the inner cam engageswith the cam follower, the operation of the mixture control valve forthe fuel-air mixture is transmitted to the air control valve, and theoperation opens the air control valve against the force of the spring.

Ideally, like the means disclosed in claim 7, which is the secondpreferred embodiment of the internal cam in claim 6, the air controlvalve has, in addition to the configuration disclosed in claim 5, avalve seat midway along the air passage and an umbrella-type valve whichcan be attached to or removed from the valve seat and which opens andcloses the air passage, the cam is fixed to the rotary shaft of themixture control valve, the cam is configured with an inner cam which isformed on the inside of the edge at a given height dropped down on theouter side along the circumference so that, if a spring exerts force inthe direction which closes the air control valve, when the edge of theinner cam engages with the cam follower, the operation of the mixturecontrol valve for the fuel-air mixture is transmitted to the air controlvalve, and the operation opens the air control valve against the forceof the spring.

If configured in this way, the air control valve is interlocked with thevalve which controls the flow rate of the fuel-air mixture. Thus whenthe engine operates under a light load, the air control valve is closedmore, and the quantity of air is reduced. This allows stable combustionwith a rich mixture. Under heavy load conditions, the air control valveis opened more, and the engine operates with a thin mixture. In this waywe can prevent noxious substances with high THC (total hydrocarbons)from being exhausted.

If configured as described above, the opening ratio of the throttlevalve and air control valve can easily be controlled in response to achange in the angular position of the mixture control valve.

Ideally, as in claim 8, in addition to the configuration disclosed inclaim 5, an air passage which connects to the outlet of the supplypassage for the fuel-air mixture runs inside the insulator. Thisinsulator is attached to the side of the cylinder downstream from theoutlet of the supply passage and the air control valve and runs in thesame direction as the air supply outlet. The air passage runs in thesame direction as the insulator.

The invention disclosed in claim 9 of this application comprises atwo-stroke cycle engine using a preceding air-layer for scavenging withan exhaust port on the side of the cylinder; a scavenging port on theside of the cylinder; a fuel passage, which supplies fuel-air mixture tothe crank chamber through the intake port on the side of the cylinderduring the time of the elevation of the piston; a scavenger passage tobe connected to the scavenging port; an air passage, which suppliesscavenging air from the air cleaner toward the inner side of the engine;an insulator, in which the fuel passage and the air passage run inparallel; a non-return valve, which is provided on the insulator facingtoward the inner side of the engine, to open or close the air passage bymeans of the negative pressure in the scavenger passage; and aconnecting passage with a small diameter to link the air passage and thefuel passages so that negative pressure in the air passage forces thefuel-air mixture in the fuel passage into the air passage.

With the invention disclosed in claim 9 of this application, thefuel-air mixture in the fuel passage is supplied to the air passagethrough a small-diameter connecting passage when the negative pressurein the air passage becomes greater than that in the fuel passage becausethe engine is idling. Thus when there is an excessive quantity of air inthe air passage during sudden acceleration, fuel-air mixture from thefuel passage will be introduced and mixed with that air.

By forcing fuel-air mixture into the airflow in the air passage, weprevent the new air supplied to the cylinder from the scavenging portfrom creating a mix with too much air. This method prevents the fuelmixture from becoming too thin during sudden acceleration of the engineand so improves the acceleration characteristics.

Furthermore, since the throttle opens during high-speed operation, thepressure differential between the fuel and air passages virtuallydisappears. Thus virtually no fuel-air mixture will flow from the fuelpassage through the small-diameter connecting passage and into the airpassage at high speeds. The method prevents fuel-air mixture fromgetting into the preceding air and so helps maintain the requiredexhaust characteristics.

When the engine is mounted obliquely, the fuel-air mixture in the fuelpassage can flow through the small-diameter connecting passage into theair passage. It will thus not collect in the bottom of the fuel passage,so that a large quantity of fuel is suddenly sucked into the cylinderwhen the engine's orientation is changed. This design, then, willprevent imperfect combustion.

With the invention disclosed in claim 9, then, we can achieve anair-layer-type scavenging two-stroke cycle engine which produces theeffect through a very simple device, i.e., a small-diameter connectingpassage between the air and fuel passages which causes the negativepressure of the air passage to draw the fuel-air mixture in the fuelpassage into the air passage. This obviates the need for a complicatedcontrol device.

In invention disclosed in claim 10, the small-diameter connectingpassage in the configuration discussed in claim 9 connects the air andfuel passages at a point downstream from the non-return valve.

If configured in this way, the connecting passage goes into the airpassage downstream from the non-return valve. When the engine is mountedobliquely, when it is, say, rotated 180°, the fuel which collects in thelower end of the fuel passage will flow through the connecting passageinto the air passage. This will prevent too much fuel from flowing intothe combustion chamber and affecting the rate of combustion, improvingboth the engine's operation and its effect.

In invention disclosed in claim 11, the small-diameter connectingpassage in the configuration discussed in claim 9 can connect the airand fuel passages at a point upstream from the non-return valve.

If configured in this way, the connecting passage goes into the airpassage upstream from the non-return valve. The diameter of the mouth ofthe connecting passage can be increased without producing a drop inengine output. This further improves the acceleration characteristicsduring sudden acceleration.

The following embodiments are preferred for the devices disclosed inclaims 10 and 11.

(1) The connecting passage is formed in the insulator gasket interposedbetween the mounting surfaces of the insulator and cylinder, or in thecarburetor gasket interposed between the mounting surfaces of thecarburetor and the insulator. If one of these configurations is adopted,it will be possible to fashion a connecting passage merely by creating aslit of the same diameter as the connecting passage in either theinsulator gasket or the carburetor gasket. Creating a connecting passagein this way is straightforward and does not require a large number ofprocesses.

(2) The connecting passage is cut into the surface of the insulatorwhere it is mounted to the cylinder.

(3) The connecting passage is cut into the surface of the cylinder whereit is mounted to the insulator.

(4) The connecting passage is formed in the carburetor gasket which goesbetween the carburetor and the insulator.

(5) The connecting passage is cut into the surface of the insulatorwhere it is mounted to the carburetor.

(6) The connecting passage is cut into the surface of the carburetorwhere it is mounted to the insulator.

(7) The connecting passage comprises a small hole in either theinsulator, the carburetor, or the cylinder, which connects the airpassage to the fuel passage.

(8) There is a non-return valve on the small hole which permits flowonly in the direction from the fuel passage to the air passage.

(9) The connecting passage is placed so that one end connects the airand fuel passages downstream from the non-return valve while the otherend connects them upstream from the valve.

If the connecting passage is configured in this way, with one endbetween the air and fuel passages downstream from the non-return valve,the fuel-air mixture in the fuel passage can flow through the connectingpassage and into the air passage when the engine is mounted obliquely.This prevents fuel from collecting at the bottom of the fuel passage andbeing sucked into the cylinder suddenly when the position of the enginechanges, so it eliminates imperfect combustion due to an excess of fuel.The other end of the connecting passage connects the air and fuelpassages upstream from the non-return valve. This makes it possible toincrease the diameter of the outlet of the connecting passage without adrop in engine output, thus improving the engine's ability to acceleratesuddenly.

The invention disclosed in claim 12 of this application comprises Atwo-stroke cycle engine with a scavenger passage which connects ascavenging port on the side of the cylinder to the crank chamber insidethe crankcase, and goes through the mounting surface where the cylinderand crankcase are attached to each other; and a removable guide with asurface forming a curved smooth channel which is attachable to thescavenger passage in the crankcase from the mounting surface, and formsa portion of the scavenger passage with the curved channel.

The guide should be configured as disclosed in claims 13 and 14.

The claim 13 comprises the configuration disclosed in claim 12. Theguide has a positioning tooth which engages with the hole in the gasketfor the surface where the cylinder and crankcase are attached to eachother.

The claim 14 comprises the configuration disclosed in claim 12. Theguide is fixed to the crankcase when its tooth engages in an indentationin the crankcase.

If the engine is configured as described above, the fuel-air mixturefrom the crank chamber in the crankcase is led into a scavenger passageone portion of which comprises a guide with a smoothly curved channel.The mixture flows through the scavenger passage formed as a smoothchannel and is supplied to the scavenging port. Because the scavengerpassage is a smoothly curved channel without any right angles, thefuel-air mixture flows smoothly and rapidly without any flow loss suchas a decrease in flow velocity as it is supplied to the scavenging port.

The guide is mounted on the crankcase in such a way that it can beremoved by pulling it away from the surface where the crankcase isattached to the cylinder along the axial direction of the cylinder. Thisobviates the need for a tooth between the upper wall of the scavengerpassage and the surface where the crankcase is attached, as was requiredin the prior art. The guide performs the same function as the tooth.When the crankcase is cast, even if a single die is used to form thescavenger passage inside the crankcase, the die can easily be removed inthe axial direction of the cylinder.

This design simplifies the casting procedure by which the scavengerpassage is formed, and it reduces the number of casting processesrequired. Because the scavenger passage can be formed using a singledie, there is no possibility that one of several dies will slip out ofposition, as sometimes happened with prior art techniques, and ruin thecasting. This design, then, improves the quality of the crankcasecontaining the scavenger passage.

If the engine is configured as disclosed in claim 13 of thisapplication, the guide has a positioning tooth which engages in a holein the gasket between the cylinder and crankcase. When the tooth engagesin the hole in the gasket, its position is assured, and the position ofthe gasket is also assured. The fact that the guide has been insertedcan be ascertained by how the gasket is positioned, so there is nochance that the guide will be forgotten.

If the engine is configured as disclosed in claim 14 of thisapplication, the crankcase has an indentation which serves as the slotinto which the tooth on the guide engages. Because the guide is fixed tothe crankcase, it will always be positioned correctly. The surface ofthe channel on the guide connects smoothly to the scavenger passage inthe crankcase.

Ideally, as is disclosed in the claim 15, which comprises theconfiguration disclosed in claim 13, the guide should have a depressionin the surface at which it is fixed to the crankcase.

If the engine is configured in this way, the slight gap between thecrankcase and cylinder where the gasket is interposed will be smaller.Less fuel will seep into the gap from the fuel-air mixture flowingthrough the scavenger passage connecting the crankcase and the cylinderthrough the common surface where they are attached to each other, whichcomprises the scavenger passage formed by the crankcase and the guide,and the scavenger passage in the cylinder. The fuel flows downwardthrough the depression formed in the guide, so even if the engine ismounted obliquely, the fuel cannot return to the scavenger passage. Thiseliminates imperfect combustion due to fuel flowing back into thescavenger passage.

Ideally, as is disclosed in the claim 16, which comprises theconfiguration disclosed in claim 12, the guide is distinguished by thefact that it is painted on.

If the guide is configured in this way, by being painted on, it willalways be perfectly plain whether or not it is there. This willeliminate the possibility that it will be forgotten during assembly. Byusing a different color for each type of machine, we can simplify ourparts control.

A preferred embodiment of the means disclosed in claim 12 of thisapplication is as follows.

The guide is formed by molding a deep-drawing sheet as a single piecewith the gasket between the cylinder and crankcase. If produced in thisway, the guide and the gasket are one piece, and deep drawing allows thechannel surface and the depression to be formed on the front and reversesides of the sheet at the same time. This reduces the parts count andthe number of assembly processes.

The invention disclosed in claim 17 of this application comprises atwo-stroke cycle engine with an exhaust port on the sidewall of thecylinder, which opens into the cylinder; a scavenging port on thesidewall of the cylinder positioned a slight distance apart in thecircumferential direction from the exhaust port, which also opens intothe cylinder; an intake port, which opens to supply fuel-air mixture tothe crankcase according to the action of the piston; and a scavengerpassage, which connects the crankcase and the scavenging port; wherein ablow-up angle (a) of the scavenger passage, which is defined by an anglebetween the upper wall which connects to the scavenging port and aperpendicular line to the axis of the cylinder, varies along thecircumferential direction of the cylinder, and if the blow-up angle in alocation nearer the exhaust port is defined as (α1) and the blow-upangle in a location nearer the intake port is defined as (α2), thenα1<α2.

Ideally, as is disclosed in claim 18, which comprises the configurationdisclosed in claim 17, the blow-up angle (α) of the scavenger passagevaries continuously from a location nearer the intake port to one nearerthe exhaust port.

Also ideally, as is disclosed in claim 19, which comprises theconfiguration disclosed in claim 17, the surface of the upper wall ofthe scavenger passage is formed so that it varies in one or more stepsfrom angle (α2) at a location nearer the intake port to angle (α1) at alocation nearer the exhaust port.

If the scavenger passage is configured as described above, when theaction of the piston causes the exhaust port and then the slightly lowerscavenging port to open, the fuel-air mixture forced into the scavengerpassage from the crankcase will flow from the scavenging port into thecombustion chamber.

Because the blow-up angle of the upper wall connecting the scavengerpassage to the scavenging port is greater at a location nearer theintake port than it is at a location nearer the exhaust port, the fuelmixture which enters the chamber from the location nearer the exhaustport will flow along the top of the piston at a high speed without beingdispersed. This will prevent it from getting caught in the exhaust gasstream and so reduce the quantity of fuel lost through the exhaust port.The fuel-air mixture which enters the chamber from the location nearerthe intake port will be flowing at a lower velocity than that nearer theexhaust port. It will be sent into the area around the spark plug in theupper part of the chamber, where it will be efficiently ignited andcombusted.

Thus this configuration prevents the fuel-air mixture from escapingunburned through the exhaust port, improves the scavenging efficiency,and increases the concentration of the fuel-air mixture which fills thecombustion chamber. This improves the combustion and increases theoutput of the engine. The fact that the fuel-air mixture is preventedfrom escaping through the exhaust port translates into a lower THC level(total hydrocarbons) in the exhaust.

If the engine is configured as disclosed in claim 19, the surface of theupper wall of the scavenger passage is formed so that it varies in stepfashion from a large angle (α2) at a location nearer the intake port toa smaller angle (α1) at a location nearer the exhaust port. When thecylinder is cast, two dies can be used with two different blow-upangles, as described above, with the angles changing at the borderbetween the dies. This will make it easy to remove the work from thedies and will reduce the number of processes necessary to produce thecylinder. Also, using dies with two different blow-up angles to form thescavenger passage is an easy and reliable way to control the blow-upangle.

The invention disclosed in claim 20 of this application comprises atwo-stroke cycle engine with a scavenging port on the side of thecylinder, which opens into the cylinder; and a scavenger passage, whichconnects the crank chamber in a crankcase and the scavenging port, andsupplies the fuel-air mixture in the crank chamber to the scavengingport; wherein the crankcase is configured in such a way that the frontand rear portions, which are separated by a block at a right angle tothe crankshaft which entails the axis of the cylinder, are fixed to eachother by mounting hardware, a scavenger passage is provided inside boththe front and rear portions of the crankcase, and the cylinder, whosescavenger passage connects to the scavenger passage in the crankcase, isfixed by mounting hardware to the mounting surface on the top of thecrankcase in such a way that the scavenger passage runs through themounting surface.

The invention disclosed in claim 27 of this application comprises atwo-stroke engine with a scavenger passage which connects the crankcaseand the scavenging port on the side of the cylinder, which opens intothe cylinder and supplies the fuel-air mixture in the crankcase to thescavenging port. This two-stroke engine is distinguished by thefollowing. In addition to having scavenger passages in both thecrankcase and the cylinder, the front and rear portions of thecrankcase, separated by a block at a right angle to the crankshaft,which entails the axis of the cylinder, are fixed to each other at theblock by mounting hardware to form a unitary crankcase. The cylinder,whose scavenger passage connects to that in the crankcase, is fixed bymounting hardware to the mounting surface at the top of the crankcase.

Ideally, as is disclosed in claim 21, which comprises the configurationin claim 20, the air passage which supplies air from the air cleaner tothe scavenger passage is formed inside the cylinder. The air passageconnects to the middle portion of the scavenger passage inside thecylinder.

If the engine is configured in this way, the crankcase is separated by ablock at a right angle to the crankshaft, and so to the axis of thecylinder, into front and rear portions, each of which has a scavengerpassage inside it. The front and rear portions of the crankcase arefixed together by mounting hardware. The cylinder, which also has ascavenger passage inside it, is fixed to the upper surface of thecrankcase by mounting hardware. The scavenger passage in the cylinder isthus linked to that in the crankcase, forming a long scavenger passagerunning through both cylinder and crankcase. Both the crankcase and thecylinder are thus compact structures with no bulges, and the scavengerpassage has a gradually curved contour without angularities.

Furthermore, a crankcase with a scavenger passage running through itsinterior can be cast in two pieces which form the front and rearportions of the engine. These can be removed from the die at the surfacewhere the front and rear pieces are separated and at the surface wherethe cylinder is mounted to the crankcase, which is perpendicular to thesurface between the pieces. This allows the die to have a simple shape,simplifies removing the work from the die, and allows the engine to beforming a long scavenger passage running through both cylinder andcrankcase. Both the crankcase and the cylinder are thus compactstructures with no bulges, and the scavenger passage has a graduallycurved contour without angularities.

Furthermore, a crankcase with a scavenger passage running through itsinterior can be cast in two pieces which form the front and rearportions of the engine. These can be removed from the die at the surfacewhere the front and rear pieces are separated and at the surface wherethe cylinder is mounted to the crankcase, which is perpendicular to thesurface between the pieces. This allows the die to have a simple shape,simplifies removing the work from the die, and allows the engine to forma long scavenger passage running through both cylinder and crankcase.Both the crankcase and the cylinder are thus compact structures with nobulges, and the scavenger passage has a gradually curved contour withoutangularities.

Furthermore, a crankcase with a scavenger passage running through itsinterior can be cast in two pieces which form the front and rearportions of the engine. These can be removed from the die at the surfacewhere the front and rear pieces are separated and at the surface wherethe cylinder is mounted to the crankcase, which is perpendicular to thesurface between the pieces. This allows the die to have a simple shape,simplifies removing the work from the die, and allows the engine to becast using fewer dies. The casting procedure is simplified and requiresfewer processes.

Ideally, as is disclosed in claim 22, which comprises the configurationin claim 20, there should be two scavenging ports along thecircumference of the cylinder. There should also be two scavengerpassages running from the outlets in the crankcase to the scavengingports. These passages should run through the block separating the halvesof the crankcase, and they should be arranged symmetrically along thefront-to-rear dimension of the engine.

If the engine is configured in this way, the scavenger passages will runthe entire length from the outlets of the crankcase to the scavengingports in the cylinder and through the separator block, and they will besymmetrical. Thus a common die can be used to cast the front and rearportions of the scavenger passages, allowing the engine to be producedwith only a few dies. The shapes of the two passages will be identical,so the cylinder will be scavenged uniformly along its circumference andfilled uniformly with the fuel-air mixture.

The following embodiments are preferred for the devices disclosed inclaims 20 through 22 of this application.

(1) The air passage branches into two passages at the inlet to thecylinder. Each of these branching air passages extends from the branchalong the length of the engine, and the two run symmetrically throughthe block separating the halves of the crankcase. They are connected tothe scavenger passages.

If the engine is configured in this way, the two branching air passagesrun in parallel through the block separating the halves of thecrankcase. Since they are identical, a common die can be used to casteach of the passages, and fewer dies need be used overall. Since theshapes of the two air passages are identical, the action of thescavenging air will be uniform along the circumference of the cylinder.

(2) The scavenger passages and branching air passages formed in thecylinder are enclosed by walls which run virtually parallel to eachother in the same direction.

If configured in this way, the walls of the scavenging and branching airpassages are integral to the cylinder and run virtually parallel to eachother. The sliding die for the cylinder can thus be a single piece,which simplifies the configuration of the die.

The invention disclosed in claim 23 of this application comprises atwo-stroke engine with a scavenger passage which connects the crankcaseand the scavenging port on the side of the cylinder; an air passageconnected to the midpoint of the scavenger passage, which suppliesscavenging air from the air cleaner to the scavenger passage; and a fuelpassage, which supplies the fuel-air mixture produced in the carburetorto the crankcase. This two-stroke engine is distinguished by thefollowing. The air cleaner has two air passages running from it inparallel, one of which is connected to the air passage, and the other ofwhich is connected to the air inlet of the carburetor to provide air forthe fuel-air mixture. A choke valve on the air cleaner opens and closesboth of these air passages.

Ideally, as is disclosed in claim 24, which comprises the configurationin claim 23, the choke valve comprises a rotary valve which, whenrotated, opens or closes the openings of the two air passages, and aknob by which the valve can be rotated.

Also, as is disclosed in claim 25, which comprises the configuration inclaim 24, the choke of the choke valve engages with the case of the aircleaner in such a way that it is free to rotate. Its front surfacecomprises a sheet which covers or uncovers the inlets of the two airpassages. A sealing ring consisting of an elastic material presses theflat surface of the valve against the openings of the inlets by elasticforce and forms a fluid seal around the valve shaft with respect to theinterior of the case.

A tapered protrusion is formed on the inner surface of the case of theair cleaner. When the rotary knob of the choke valve strikes theprotrusion, the flat surface of the valve is pressed against the openingof either the first or the second of the two air passages.

If the choke is configured in this way, when the knob of the choke valveis turned to start up the engine, the second air passage, which connectsthe air cleaner to the air inlet of the carburetor, is completelyclosed, and only the choke hole is open. The first air passage, whichconnects the air cleaner to the air passage supplying the preceding air,is also completely closed when the engine is started up.

When the choke is adjusted in this way, the air from the air cleanerwhich is supplied to the carburetor via the second air passage isconstricted by the choke hole. The fuel-air mixture produced in thecarburetor is supplied via the crankcase, scavenger passage andscavenging port to the combustion chamber.

Since the first air passage from the air cleaner is completely closed bythe choke at this time, the fuel-air mixture from the carburetor issupplied to the combustion chamber without any preceding air. Thus thechamber is filled with a rich fuel-air mixture. This improves thestart-up characteristics of the engine.

The flat surface of the rotary choke of the choke valve covers anduncovers the openings of the two air passages. The elastic force of theO-ring or other sealing ring which is inserted around the shaft of thechoke causes the flat surface to press against the openings. The chokevalve thus completely and reliably closes the opening of the passage forthe preceding air. This promotes the production of a rich fuel-airmixture as described above and allows a high negative pressure to bemaintained.

This invention provides both a sealing ring as described above and aprotrusion on the outside of the case of the air cleaner to serve as astop for the choke, and rotating the choke valve from its initialposition to its normal operating position will switch between the twoaforesaid air passages. When the choke valve is rotated, its knob easilygoes over the protrusion against the force of the sealing ring. Themoderate friction improves the operating feel of the choke valve. Whenthe choke valve is released, the elastic force of the sealing ring andthe force of the protruding stop automatically hold the choke valve inplace on the flat portion of the case in such a way that it cannot goback. This insures easy operation.

There is, as is disclosed in claim 25, a tapered protrusion on the innersurface of the case of the air cleaner. When the choke valve is rotatedtoward starting position, the knob of the choke valve goes over theprotrusion. When this occurs, the elastic force generated by thedeflection of the valve causes its flat surface to push down on theopening of the passage for the preceding air. This improves the sealingfunction of the flat surface, and thus improves the start-upcharacteristics.

The invention disclosed in claim 26 of this application is a two-strokeengine which is distinguished by the following. It has a rotary valveinstalled on the case in such a way that it is free to rotate which,when rotated, opens and closes the two air passages; and a rotary knobwhich operates the valve. The front surface of the valve comprises asheet which covers or uncovers the inlets of the two air passages. Asealing ring consisting of an elastic material presses the flat surfaceof the valve against the openings of the inlets by elastic force andforms a fluid seal around the valve shaft with respect to the interiorof the case.

A rotary valve configured in this way is not limited in its applicationto use as a choke valve for the air cleaner of a two-stroke engine. Itcan be used in a wide range of applications which require switchingbetween two fluid passages by operating a rotary valve.

To summarize, the effects of the invention disclosed above are asfollows.

(1) With the inventions disclosed in claims 1 through 8 of thisapplication, all scavenger passages and air passages are formed insidethe crankcase and the cylinder. This obviates the need for externalpipes to serve as air passages as well as their mounting hardware. Fewerparts and assembly processes are required, and the engine can be madelighter and smaller.

The scavenger passage is formed of two passages, one created by walls inthe crankcase and the other by similar walls in the cylinder. The resultis a long scavenger passage which can be filled with air for thescavenging operation. Since the fuel-air mixture is supplied only afterthe crankcase has been thoroughly scavenged by this air, no fuel is lostthrough the exhaust port, and fuel wastage is minimized.

If configured as disclosed in claim 2, the outlet of the scavengerpassage on the side of the crankcase opens into the crank chamber. Thegap between the side of the crankcase and the crank web is reduced, anda disk valve is formed by the outlet on the crankcase and the crank web.This controls the velocity of the air forced in through the scavengingport and reduces the quantity of fuel-air mixture which becomes trappedin the exhaust gas stream.

If configured as disclosed in claim 4, the branching air passage and thebranching passage to the scavenging port in the cylinder are bothenclosed by walls that run parallel in the same direction. This allowsthe sliding die for the scavenger passage to be one piece in the processof casting the cylinder. The die can have a simpler shape and will becheaper to produce.

If the engine is configured as disclosed in claims 5 through 8, a valvecontrols the air flow supplied to the combustion chamber via thescavenging port of the engine. This air control valve is interlockedwith the fuel-mixture control valve through a cam mechanism whichrotates with the fuel-mixture valve. This allows the opening and closingof the air and fuel control valves to be controlled in relation to eachother.

Thus when the engine is operating under a light load, the air controlvalve can be fully closed or opened only slightly so that the fuelmixture is richer and stable combustion can be maintained. When theengine is operating under a heavier load, the air control valve can beopened or closed proportionally with the fuel mixture control valve toproduce a thinner mixture. We can thus provide an air-layer-typescavenging two-stroke engine in which the overall richness of themixture can be kept at the appropriate concentration, the rate of fuelconsumption is reduced, and the quantity of pollutants in the exhaustgas is lowered.

The opening of the mixture control valve which controls the flow rate ofthe fuel-air mixture is interlocked via a cam mechanism to the openingof the air control valve. The angular ratio of the mixture control valveto the air control valve can be set as desired; it will remain constantregardless of the angular position of the mixture control valve. Thisdesign, then, allows the user to select the most advantageous ratio.

(2) With the invention disclosed in claim 9, the air passage and thefuel mixture passage are linked by a small-diameter connecting passage.Thus the fuel mixture in the fuel passage can be supplied via thissmall-diameter connecting passage to the air passage. In this way morefuel-air mixture can be added to the air flowing through the airpassage. This prevents the new air supplied to the cylinder from thescavenging port from creating too thin a mix when the engine acceleratessuddenly, and thus improves the acceleration characteristics.

When the engine is operating at high speed, there is virtually nopressure differential between the fuel passage and the air passage.There will therefore be almost no fuel-air mixture flowing via thesmall-diameter connecting passage from the fuel passage to the airpassage, and thus no fuel in the layer of preceding air. This willinsure that the required exhaust specifications can be maintained.

When the engine is mounted obliquely, the fuel-air mixture in the fuelpassage can flow into the air passage via the connecting passage. Thisprevents fuel from collecting in the lowest portion of the fuel passageso that a large quantity of fuel is suddenly sucked into the cylinderwhen the engine's orientation is changed. This design, then, willprevent the imperfect combustion which would occur if there were excessfuel in the chamber.

Providing a small-diameter connecting passage between the air and fuelpassages obviates the need for a control device with a complicatedconfiguration. It allows us to realize an air-layer-type scavengingtwo-stroke engine which can achieve the same effect with an extremelysimple device.

One end of the connecting passage connects the air and fuel passagesdownstream from the non-return valve. The effect of this when the engineis mounted obliquely is that it will prevent an excess of fuel in thecombustion chamber which will result in imperfect combustion. The otherend of the connecting passage connects the air and fuel passagesupstream from the non-return valve. This makes it possible to increasethe diameter of the outlet of the connecting passage without a drop inengine output, thus improving the engine's ability to acceleratesuddenly.

(3) With the invention disclosed in claim 12, a surface of a guide whichforms the shape of a channel connects smoothly with the scavengerpassage in the crankcase. The resulting scavenger passage is graduallycurved with no right angles, so the fuel-air mixture which moves throughit does not experience any loss of flow through deceleration, but issupplied to the scavenging port smoothly and at a high velocity. Thisimproves the engine output.

The guide is mounted on the crankcase in such a way that it can beremoved by pulling it away from the surface where the crankcase isattached to the cylinder along the axial direction of the cylinder. Theguide fulfills the function of the projection that was used in the priorart. When the crankcase is cast, even if a single die is used to formthe scavenger passage inside the crankcase, the die can easily beremoved in the axial direction of the cylinder.

This design simplifies the casting procedure by which the scavengerpassage is formed, and it reduces the number of casting processesrequired. Because the scavenger passage can be formed using a singledie, there is no possibility that one of several dies will slip out ofposition, as sometimes happened with prior art techniques, and ruin thecasting. This design, then, improves the quality of the crankcasecontaining the scavenger passage.

If the engine is configured as disclosed in claim 13, the guide has apositioning tooth which engages with the hole in the gasket. When thetooth engages in the gasket hole, its position is guaranteed, and so isthat of the gasket. The fact that the guide is in place can be knownfrom how the gasket is seated, so there is no chance of forgetting theguide.

If the engine is configured as disclosed in claim 14, the guide is fixedto the crankcase when its tooth engages in an indentation in thatcrankcase. In this way the guide can be positioned with completeaccuracy so that its surface which forms a channel can connect smoothlyto the scavenger passage in the crankcase.

If the engine is configured as disclosed in claim 15, even if the fuelin the fuel-air mixture flowing through the scavenger passage connectingthe crankcase and the cylinder through the common surface where they areattached to each other seeps into the gap between the surfaces where thegasket is inserted, the fuel will flow downward through the depressionformed in the guide. Thus even if the engine is mounted obliquely, thefuel cannot return to the scavenger passage. This eliminates imperfectcombustion due to fuel flowing back into the scavenger passage.

(4) If the engine is configured as disclosed in Claim 17, the blow-upangle of the upper wall of the scavenger passage which connects it tothe scavenging port is greater at a location nearer the intake port thanat one nearer the exhaust port. The fuel mixture which enters thechamber from the location nearer the exhaust port will flow along thetop of the piston at a high speed without being dispersed. This willprevent it from getting caught in the exhaust gas stream and so reducethe quantity of fuel lost through the exhaust port. The fuel-air mixturewhich enters the chamber from the location nearer the intake port willbe flowing at a lower velocity than that nearer the exhaust port. Itwill be sent into the area around the spark plug in the upper part ofthe chamber, where it will be efficiently ignited and combusted. Thescavenging efficiency is improved, the fuel-air mixture which fills thecombustion chamber has a higher concentration, and the combustion isimproved, resulting in greater engine output. Also, preventing thefuel-air mixture from escaping reduces the level of THC (totalhydrocarbons) in the exhaust.

If the engine is configured as disclosed in claim 19, the surface of theupper wall of the scavenger passage is formed so that it varies in stepfashion from a large blow-up angle at a location nearer the intake portto a smaller blow-up angle at a location nearer the exhaust port. Whenthe cylinder is cast, two dies can be used with two different blow-upangles, with the angles changing at the border between the dies. Thiswill make it easy to remove the work from the dies and will reduce thenumber of processes necessary to produce the cylinder.

Also, using dies with two different blow-up angles to form the scavengerpassage is an easy and reliable way to control the blow-up angle.

(5) With the inventions disclosed in claims 20 and 27 of thisapplication, the crankcase is divided into front and rear portions, eachof which has a scavenger passage inside it. These two portions of thecrankcase are fixed to each other by mounting hardware. The cylinder,whose scavenger passage connects to that in the crankcase, is fixed bymounting hardware to the mounting surface at the top of the crankcase.This results in a long scavenger passage running through both cylinderand crankcase. Both the crankcase and the cylinder are thus compactstructures with no bulges, and the scavenger passage has a graduallycurved contour without angularities.

Furthermore, a crankcase with a scavenger passage running through itsinterior can be cast in two pieces which form the front and rearportions of the engine. These can be removed from the die at the surfacewhere the front and rear pieces are separated and at the surface wherethe cylinder is mounted to the crankcase, which is perpendicular to thesurface between the pieces. This allows the die to have a simple shape,simplifies removing the work from the die, and allows the engine to becast using fewer dies. The casting procedure is simplified and requiresfewer processes, with the result that the cost is lower.

If the engine is configured as disclosed in claim 22, the scavengerpassages will run the entire length from the outlets of the crankcase tothe scavenging ports in the cylinder and through the surfaces where thetwo portions meet, and they will be symmetrical. Thus a common die canbe used to cast the front and rear portions of the scavenger passages,allowing the engine to be produced with fewer dies. The shapes of thetwo passages will be identical, so the cylinder will be scavengeduniformly along its circumference and filled uniformly with the fuel-airmixture.

Furthermore, the two branching air passages run symmetrically throughthe surface between the two halves of the crankcase. If the passages areconfigured in this way, a common die can be used to cast both of them,so fewer dies need be used overall. Since the shapes of the two airpassages are identical, the action of the scavenging air will be uniformalong the circumference of the cylinder.

Also, the walls of the scavenging and branching air passages can beformed integrally to the cylinder and virtually parallel with eachother. If configured in this way, the sliding die for the cylinder canbe a single piece. This simplifies the configuration of the die andreduces the cost of producing it.

(6) With the invention disclosed in claim 23, the first air passage fromthe air cleaner is completely and reliably closed by the choke valvewhen the engine is being started up. Thus the supply of scavenging airto the combustion chamber is cut off, and only the fuel-air mixture fromthe carburetor is supplied to the combustion chamber. Thus the chamberis filled with a rich fuel-air mixture. This improves the start-upcharacteristics of the engine.

The flat surface of the rotary choke of the choke valve covers anduncovers the openings of the two air passages. The elastic force of theO-ring or other sealing ring which is inserted around the shaft of thechoke causes the flat surface to press against the openings. Ifconfigured in this way, the choke valve completely and reliably closesthe opening of the passage for the preceding air. This promotes theproduction of a rich fuel-air mixture as described above and allows ahigh negative pressure to be maintained.

In addition to the sealing ring, a protrusion is provided on the outsideof the case of the air cleaner to serve as a stop for the choke. If thechoke valve is configured in this way, rotating it from its initialposition to its normal operating position will switch between the twoaforesaid air passages. When the choke valve is rotated, its knob easilygoes over the protrusion against the force of the sealing ring. Themoderate friction improves the operating feel of the choke valve. Whenthe user releases the choke valve after operating it, the elastic forceof the sealing ring and the force of the protruding stop automaticallyhold the choke valve in place on the flat portion of the case in such away that it cannot go back. This insures easy operation.

Furthermore, as is disclosed in claim 25, there is a tapered protrusionon the inner surface of the case of the air cleaner. When the chokevalve is rotated toward starting position, the knob of the choke valvegoes over the protrusion. When this occurs, the elastic force generatedby the deflection of the valve causes its flat surface to push down onthe opening of the passage for the preceding air. This improves thesealing function of the flat surface, and thus improves the start-upcharacteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section taken at a right angle to the crankshaft whichindicates the center passage of the cylinder in an air-layer-typescavenging two-stroke engine which is the first preferred embodiment ofthis invention.

FIG. 2 is a perspective drawing showing the arrangement of thescavenging and air passages in the first preferred embodiment.

FIG. 3 corresponds to FIG. 2, and shows the operation of the enginewhich is the first preferred embodiment.

FIG. 4 shows the relationship between the crank web and the scavengerpassage in the first preferred embodiment.

FIG. 5 is a cross section taken along passage A-A in FIG. 1.

FIG. 6 is a cross section of the air control valve and its surroundingarea in the second preferred embodiment of this invention.

FIG. 7 is a horizontal cross section of the air control valve in thesecond preferred embodiment of this invention.

FIG. 8 is the view of the air control valve seen from line B-B in FIG.6.

FIG. 9 is a graph showing an example of the relationship between thethrottle valve and the air control valve.

FIG. 10 corresponds to FIG. 6 and shows the third preferred embodimentof this invention.

FIG. 11 is a cross section which corresponds to FIG. 7 and shows thethird preferred embodiment of this invention.

FIG. 12 eighth preferred embodiment of this invention.

FIG. 13 is a frontal view of the carburetor gasket in the fourthpreferred embodiment of this invention.

FIG. 14 is a view of the end of the carburetor gasket on the insulator(from line A0-A0 in FIG. 12) in the fourth preferred embodiment of thisinvention.

FIG. 15 is a magnified cross section of the area where the insulator andcylinder are connected (an enlargement of portion Z in FIG. 12) in thefourth preferred embodiment of this invention.

FIG. 16 is a plan view of the insulator gasket in the fourth preferredembodiment of this invention.

FIG. 17 is a view of the end of the insulator gasket on the insulator(from line B0-B0 in FIG. 15) in the fourth preferred embodiment of thisinvention.

FIG. 18 is a view of the connecting passage in the fourth preferredembodiment of this invention which corresponds to FIG. 15.

FIG. 19 is a cross section taken at a right angle to the crankshaft andshowing the scavenger passage in a two-stroke engine which is the fifthpreferred embodiment of this invention.

FIG. 20 is the view from line A1-A1 in FIG. 19.

FIG. 21 is a frontal view of the guide in the fifth preferredembodiment.

FIG. 22 is the view from arrow B1 in FIG. 19.

FIG. 23 is a cross section taken along the axis of the cylinder andshowing the cylinder in an air layer-type two-stroke engine which is thesixth preferred embodiment of this invention.

FIG. 24 is a view of the seventh preferred embodiment of this inventionwhich corresponds to that in FIG. 23.

FIG. 25 are cross sections of the scavenger passages in the cylinders ofthe sixth and seventh preferred embodiments. (A) is taken along lineA2-A2 in FIGS. 23 and 24; (B) is taken along line B2-B2.

FIG. 26 is a perspective drawing of the scavenging port in the seventhpreferred embodiment.

FIG. 27 is a cross section taken along the axis of the cylinder in anair layer-type scavenging two-stroke engine in which the sixth andseventh preferred embodiments of this invention have been implemented.

FIG. 28 is an exploded perspective drawing of an air layer-typescavenging two-stroke engine which is the eighth preferred embodiment ofthis invention.

FIG. 29 is a cross section taken along the crankshaft of a two-strokeengine which is the eighth preferred embodiment of this invention.

FIG. 30 shows the configuration of the scavenging and air passages inthe eighth preferred embodiment of this invention.

FIG. 31 is a cross section taken at a right angle to the crankshaftwhich describes the axis of the cylinder in an air layer-type scavengingtwo-stroke engine in which this invention has been implemented.

FIG. 32 is a cross section taken at a right angle to the crankshaftwhich shows the configuration of the air cleaner and vacuum device in atwo-stroke engine which is the ninth preferred embodiment of thisinvention.

FIG. 33 is the view from arrow 3 in FIG. 32.

FIG. 34 is the view from line B3-B3 in FIG. 32.

FIG. 35 is the view from line C3-C3 in FIG. 32.

FIG. 36 is a cross section of the air cleaner cover and choke valve (acloser view of area Z in FIG. 32).

FIG. 37 is the view from line D3-D3 in FIG. 36.

FIG. 38 is a cross section taken along line E-E in FIG. 37.

FIG. 39 is an example of the prior art which corresponds to FIG. 19.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In this section we shall give a detailed explanation of a number ofpreferred embodiments of this invention with reference to the drawings.Whenever the shapes, relative positions and other aspects of the partsdescribed in the embodiments are not clearly defined, the scope of theinvention is not limited only to the parts shown, which are meant merelyfor the purpose of illustration.

FIGS. 1 through 5 show the first preferred embodiment of this invention.2 is the cylinder, 4 the piston, 6 the crankshaft, 6a the crank webcomprising the crankshaft 6, 5 the crankcase, 3 the connecting rod whichconnects piston 4 and crankshaft 6, 7 the cylinder head, 8 the sparkplug, 11 the air cleaner and 12 the carburetor. 25 is the combustionchamber. 5a is the crank chamber formed inside crankcase 5. 15b and 15are the passage for the fuel-air mixture which connects the carburetor12 and crank chamber 5a. 13a is the exhaust port which opens into theside of cylinder 2. It is connected to exhaust pipe 13.

9a are two scavenging ports to the right and left of exhaust port 13a oncylinder 2, oriented at virtually a right angle to exhaust port 13a. Ascan be seen in FIG. 2, the scavenging ports 9a are connected to thecrank chamber 5a via two branching scavenger passages 109e, which aremounted obliquely on cylinder 2; two scavenger passages 109f, which areat the point where the various scavenger passages meet; arc-shapedscavenger passages 109d, which are enclosed by walls on either side ofcrankcase 5; and outlets 109b.

As can be seen in FIG. 5, the end surfaces 109g of the outlets 109b andthe end surfaces 6d of crank webs 6 approximate each other in thedirection of crankshaft 60, leaving only a microscopic gap, so that theends of the outlets can be opened and closed by the action of crank webs6a of crankshaft 6. At a right angle to crankshaft 60, as can be seen inFIG. 4, the outlets 109b of the scavenger passages are progressivelyuncovered by crank webs 6a as crankshaft 6 rotates in direction N. Thelower portions of outlets 109b 6a are tapered so that initially a smallpart of the openings is uncovered and then progressively a larger andlarger portion of them.

10 is an air supply chamber in the side of cylinder 2. Its upstream sideis connected to air passage 10b in insulator 30, which will be discussedshortly. Its downstream side is connected to the two branching airpassages 10a. The branching air passages 10a, as is shown in FIG. 2,connect to scavenger passages 109f and branching scavenger passages109e.

The air supply chamber 10 has non-return valves 16 on the outlets tobranching air passages 10a on its right and left, which permit air toflow only toward branching air passages 10a.

As can be seen in FIGS. 2 and 3, the branching air passages 10a andscavenger passages 109e are formed virtually symmetrically with respectto the axis 50 of the cylinder by walls 109h and 109i, which extendoutward from the sides of the cylinder 2 and are integral to it. Thewalls 109h and 109i are parallel to each other. The single die whichforms them can be removed by pulling it sideward away from the cylinder.

30 is an insulator to thermally isolate the engine body from the vacuumsystem. The insulator 30 is bolted to the side of cylinder 2. The airpassage 10b is in the upper portion of the insulator 30, and fuelpassage 15b is in its lower portion.

The upstream side of the fuel passage 15b is connected to control valve14 on the carburetor, which controls the rate of flow of the fuel-airmixture. The downstream side is connected to the interior of thecylinder (combustion chamber 25) via fuel supply inlet 15a.

110 is an air passage which is integral to carburetor 12. It connectsair cleaner 11 and insulator 30. Control valve 20 varies the size of theopening of the air passage 110. The control valve 20 is interlocked withmixture control valve 14 on carburetor 12.

When an air layer-type scavenging two-stroke engine configured in thisway operates, the explosive force inside combustion chamber 25 pushespiston 4 downward and opens exhaust port 13a. The combustion gases(i.e., the exhaust gas) in the chamber 25 go out exhaust port 13a intoexhaust pipe 13 and are released to the exterior through the muffler(not pictured).

When piston 4 goes down further, the scavenging ports 9a to its left andright open. The air which has accumulated in branching scavenger passage109 flows into combustion chamber 25, as indicated by the arrow in FIG.3, and pushes the combustion gases toward exhaust port 13a.

Next, the fuel-air mixture stored in crank chamber 5a flows intocombustion chamber 25 via outlets 109b of the scavenger passages,scavenger passages 109d and branching scavenger passages 109e.

When piston 4 goes down, as can be seen in FIG. 3, and reaches bottomdead center, exhaust port 13a and scavenging ports 9a open, and thesupply of air and fuel-air mixture to combustion chamber 25 is completedor attempts to be completed. When piston 4 rises from bottom deadcenter, it closes scavenging ports 9a, and the interior of crank chamber5a becomes a closed space. As the space begins to expand, its pressurebegins to drop.

When piston 4 rises further, exhaust port 13a closes, and the fuel-airmixture in combustion chamber 25 begins to be pressurized. As piston 4rises, the volume inside crank chamber 5a increases, which furtherreduces the pressure in the crankcase. As can be seen in FIG. 2, whenpiston 4 rises further, fuel supply inlet 15a on the side of cylinder 2opens, and the fuel-air mixture generated in carburetor 12 andcontrolled by valve 14 is supplied to crank chamber 5a through fuelpassages 15b and 15, as indicated by the arrows in FIG. 2.

The drop in pressure inside the crank chamber 5a is communicated viaoutlets 109b, scavenger passages 109d and branching scavenger passages109e to branching air passages 10a on the left and right. Non-returnvalve 16 opens, and the air supplied to air supply chamber 10 via valve16 through a process we shall discuss shortly flows into crank chamber5a.

The various pairs of passages which run from the scavenging ports 9a tocrank chamber 5a, namely, branching scavenger passages 109e, andscavenger passages 109f and 109d, together form two long scavengerpassages. The air supplied to the scavenger passages must fill theirentire length before it is admitted into crank chamber 5a.

When piston 4 reaches the vicinity of top dead center, spark plug 8discharges a spark in combustion chamber 25. This ignites thepressurized fuel-air mixture and combustion occurs. The pressuregenerated by this combustion pushes piston 4 down, which generatesrotary torque in crankshaft 6.

When piston 4 goes down and exhaust port 13a opens, the combustion gasesin combustion chamber 25 flow through exhaust port 13a into exhaust pipe13. They are exhausted to the exterior through the muffler (notpictured).

When piston 4 begins to drop, the gases in crank chamber 5a arepressurized by its reverse side. When piston 4 drops further, theoutlets of scavenging ports 9a on either side of it open. The fuel-airmixture supplied to crank chamber 5a as described above is sucked intocombustion chamber 25 from scavenging ports 9a via outlets 109b,scavenger passages 109d and 109f, and branching scavenger passagess109e. The combustion gases (i.e., the exhaust gas) in the combustionchamber 25 is pushed out through exhaust port 13a in the scavengingoperation.

When the chamber is to be scavenged, the non-return valve 16 describedabove is opened, and an appropriate quantity of air is allowed to fillscavenger passages 109d and 109f and branching scavenger passages 109e.Thus at the completion of a specified time interval from the beginningof scavenging, everything between scavenging ports 9a and combustionchamber 25 will have been scavenged by air. Only then is the fuel-airmixture in crank chamber 5a forced into combustion chamber 25 fromscavenging ports 9a via scavenger passages 109d and branching passages109e.

When the process is executed repeatedly, the engine operates and poweris generated.

As is shown in FIGS. 1 through 3, in this sort of preceding airlayer-type scavenging two-stroke engine, scavenger passages 109d incrankcase 5 are symmetrical with respect to the axis 50 of the cylinderon either side of the crankcase, and scavenger passage outlets 109b openinto crank chamber 5a symmetrically with respect to axis 50 of thecylinder.

And as can be seen in FIG. 5, crank webs 6a form disk valves overoutlets 109b into crankcase 5, with a small gap between the ends 109g ofthe outlets 109b and the ends 6d of crank webs 6a.

FIG. 4 shows how an outlet 109b is progressively uncovered when a crankweb 6a rotates in crank chamber 5a. The position of the outlets 109bvaries along the direction of rotation N of crankshaft 6. The timing atwhich the scavenging air is forced through scavenging ports 9a alsovaries. As can be seen in FIG. 4, the openings of outlets 109b aretapered so that their size progressively increases as crank webs 6arotate. By controlling the velocity of the scavenging air entering viascavenging ports 9a, we can further reduce the quantity of fuel-airmixture which gets caught in the exhaust gas stream.

In the preceding air layer-type scavenging two-stroke engine which isthe first preferred embodiment of this invention, branching air passages10a on cylinder 2 and branching scavenger passages 109e to scavengingports 9a on cylinder 2 are surrounded by parallel walls 109h and 109i,as can be seen in FIG. 2. The sliding die used to cast the cylinder canthus be a single piece. The configuration of the die is simpler, andthis reduces its cost.

FIGS. 6 through 8 show the second preferred embodiment of a precedingair layer-type scavenging two-stroke engine according to this invention.In this embodiment, the air control valve 20 of the two-stroke engine ofthe first embodiment is replaced by a modified valve 20 which has thefollowing improvements.

In FIGS. 6 through 8, 11 is the air cleaner, 12 the carburetor, 10e theair passage in carburetor 12, and 14 the fuel mixture control valve ofcarburetor 12. 15 and 15b are the fuel mixture passages, and 15a is thesupply inlet for the fuel mixture on cylinder 2.

20 is the air control valve. 45 is an air pipe connecting the airpassage 10e of the air cleaner 11 to a separate outlet. 35 is the airsupply pipe in the insulator attached to the cylinder 2. 36 is a jointpipe for intake air. It is attached to air supply pipe 35 in theinsulator and connected to the outlet of the air passage 110.

As is shown in FIG. 7, the scavenging air, which is conducted through adifferent outlet on the air cleaner 11 than is the air passage 10e,flows through air passage 110 in air pipe 45. It goes through airpassage 10b, which is surrounded by the joint pipe 36 for air intake andair supply pipe 35 of the insulator. It passes from air valve 37 ofcontrol valve 20, which will be discussed shortly, and valve seat 35a.It flows through air supply chamber 10 in cylinder 2 by way ofnon-return valve 16, and is sent to branching air passage 10a, as shownin FIG. 1.

20 is an air control valve. The valve 20 is umbrella-shaped and isinstalled on joint pipe 36 for air intake. Air valve 37, which isattached to the end of the valve stem 39 in such a way that it can movealong the stem, engages in valve seat 35a in such a way that it canfreely be attached or removed. Cam follower 38 is on the end of thevalve stem 39 which juts out from joint pipe 36. Compressive spring 41exerts force in the direction which is downstream in terms of the airflow through the air valve 37, that is, it exerts pressure on air valve37 against valve seat 35a.

Portion 124c is bent downward on the end of control lever 124, which isattached to the rotary shaft of fuel mixture control valve 14 of thecarburetor 12. Cam 124a, which has a fan-shaped cross section, is formedon the bent portion 124c. The internal cam 124a engages with camfollower 38, which is on the other end of the air valve 37, to open andclose that valve. The internal cam 124a engages with the cam follower 83so as to open air valve 37 in the direction opposite the flow of air,through the cam follower 38, against the force of compressive spring 41.

As is well known, the control lever 124 adjusts the opening of mixturecontrol valve 14; i.e., it adjusts the rate of flow of the fuel-airmixture. Swivel 125 is mounted so that it can rotate on operating end124b of the side of the control lever 124 which is opposite the internalcam 124a. Control lever 124 is operated by control cord 142, which isconnected from the exterior to the swivel 125.

In the second preferred embodiment, the pressure in crank chamber 5adrops when piston 4 rises. When a negative pressure is achieved, it iscommunicated from crank chamber 5a to air supply chamber 10 viascavenger passages 109b, 109d and 109f and branching air passages 10a.When the air supply chamber 10 goes to negative pressure, non-returnvalve 16 opens.

When the control lever 124 is operated, internal cam 124a rotates andcam follower 38 of air control valve 20 is pulled leftward in FIGS. 6and 7 against the spring force of compressive spring 41. This opens airvalve 37. Scavenging air from air cleaner 11 flows into air supplychamber 10 through air pipe 45, air passages 110 and 10a, air valve 37and non-return valve 16.

From the air supply chamber 10, just as in the first embodiment, thescavenging air flows into crank chamber 5a by way of branching airpassages 10a, scavenger passages 109f and 109d and scavenger passageoutlets 109b as indicated by arrows in FIGS. 2 and 3. It accumulates inthe passages and the crank chamber 5a, and scavenges them in the sameprocess as in the first embodiment.

Generally, in two-stroke engines, mixture control valve 14 will beadjusted under a partial load to control the rate of flow of thefuel-air mixture supplied to combustion chamber 25 through crank chamber5a. In this way a nearly constant concentration of fuel-air mixture canbe maintained over a wide range of operation.

In the air layer-type scavenging two-stroke engine related to thisinvention, as has been discussed above, in the scavenging process air issupplied to combustion chamber 25 through air supply chamber 10 insteadof fuel-air mixture. If the concentration of the fuel-air mixture has agiven concentration, and a quantity of air proportional to the quantityof fuel-air mixture is supplied to combustion chamber 25, the overallrichness of the mixture can be kept constant.

In the second preferred embodiment of this invention, mixture controlvalve 14, which controls the flow rate of the fuel-air mixture, and aircontrol valve 20, which controls the flow rate of the scavenging airsent into combustion chamber 25 through air supply chamber 10, areinterlocked through internal cam 124a of lever 124, which adjustsmixture control valve 14, and cam follower 38. By selecting the profileof internal cam 124a, we can attain an appropriate ratio for the degreeof openness of mixture control valve 14 and air control valve 20 (i.e.,an appropriate ratio of the flow rate of the fuel-air mixture to that ofthe scavenging air).

The graph shown in FIG. 9 is an example of an appropriate relationshipbetween the degree of opening of mixture control valve 14 and aircontrol valve 20. In this figure, R1, which is shown by a solid line, isan example in which combustion is stabilized under light loadconditions, including idling, which occur until valve 14 reaches itsfixed Z point of opening, by supplying a relatively rich mixture. Inthis region, i.e., the region in which valve 14 has not yet reached itsZ point, air control valve 20 is completely closed. Beyond the Z point,the degree of openness of valves 14 and 20 are set proportionally toeach other. R2, which is shown by a broken line, is an example in whichair control valve 20 is less open than in R1 while mixture control valve14 is only partly open.

In the second preferred embodiment, mixture control valve 14, whichcontrols the flow rate of the fuel-air mixture, and air control valve 20are interlocked through internal cam 124a, which is connected to thecontrol lever 124, and cam follower 38. Thus the opening ratio of themixture control valve 14 and air control valve 20 can easily be set inresponse to a change in the angular position of mixture control valve14. In other words, as can be seen in FIG. 9, while the engine is undera light load, as when idling, until it reaches the Z point, air controlvalve 20 is completely closed. Fuel-air mixture whose flow rate iscontrolled by mixture control valve 14 is sent into combustion chamber25 to fill the chamber both for scavenging and combustion.

Once the opening of mixture control valve 14 reaches the Z point in FIG.9, internal cam 124a of control lever 124 draws air valve 37 out to openit, and air control valve 20 moves into the open position.

When the load on the engine increases and mixture control valve 14 opensfurther, air control valve 20 operates with the rotation of internal cam124a of control lever 124, which follows the rotation of valve 14, andopens proportionally to the valve 14. The flow rate of air increases,and the engine is run with the weak mixture that a heavy load demands.

Other aspects of the configuration are identical to correspondingaspects of the first preferred embodiment, pictured in FIGS. 1 through5. The same parts have been numbered in the same way as in thesefigures.

FIGS. 10 and 11 show an air layer-type scavenging two-stroke enginewhich is the third preferred embodiment of this invention. Thisembodiment concerns the valve which controls the quantity of scavengingair and its operating mechanism, as in the second embodiment. It differsfrom the second embodiment in that the internal cam on control lever 124is formed at a given height on the inner edge of the fan-shapedperiphery of the control lever.

In FIGS. 10 and 11, the scavenging air conducted out of air cleaner 11through a different outlet than the one to air passage 10e passesthrough air passage 10b, which is surrounded by joint pipe 48 and intakepipe 47 in the insulator. It flows through the opening of air valve 37and valve seat 47a in air control valve 20. It goes through non-returnvalve 16 and air supply chamber 10 in cylinder 2 and is sent tobranching air passage 109 (See FIG. 1).

Control lever 124 is fixed to the rotary stem of mixture control valve14 on the carburetor 12. On the end of control lever 124, in contrast tothe configuration of the second embodiment, is portion 127c, which isbent upward. This forms internal cam 127a, whose horizontal crosssection is fan-shaped. The internal cam 127 is interlocked with camfollower 38 of air valve 37 so that it opens and closes that air valve.The internal cam 127a is installed so that it opens air valve 37 throughthe action of cam follower 38 in the direction opposite the air flow,i.e., in the direction in which it exerts pressure on compressive spring41.

Other aspects of the configuration and operation of this engine areidentical to corresponding aspects of the second embodiment, and havebeen given the same numbers.

FIGS. 12 through 18 show the fourth preferred embodiment of thisinvention. In this embodiment, the respective supply systems for thepreceding air and the fuel mixture have been improved. The basicconfiguration of this engine is identical to that of the first throughthird embodiments. Corresponding parts have been given the same numbers.

In FIGS. 12 through 18, 41 is an insulator gasket, which is placedbetween the surfaces where the insulator 30 and cylinder 2 are mountedto each other; 43 is a carburetor gasket, which is placed between thesurfaces where the insulator 30 and carburetor 12 are mounted to eachother.

The carburetor gasket 42, which is shown in FIG. 14, has two parallelpassages running through it, air passage 10b in its upper portion andfuel-mixture passage 15 in its lower portion, in the same positions asin the insulator 30. Connecting passage 43 is a slit which connects theair passage 10b and fuel-mixture passage 15. 46 are bolt holes.

The insulator gasket 41, which is shown in FIG. 16, has the same twoparallel passages running through it, air passage 10b in its upperportion and fuel-mixture passage 15 in its lower portion, at the samepitch as in the insulator 30. Connecting passage 44 is a slit which runsbetween and connects the air passage 10b and fuel-mixture passage 15. 61is a space for the non-return valve 16; 49 are bolt holes.

Connecting passage 44 in the insulator gasket 41, as can be seen in FIG.15, connects fuel-mixture passage 15 and air supply chamber 10 in alocation which is downstream from flat surface 45 of the non-returnvalve 16. Its diameter, that is, the diameter of its passage, is smallerthan that of connecting passage 43 in carburetor gasket 42. Creating theconnecting passage 44 minimizes any drops in output.

Instead of providing connecting passages 44 and 43 in the insulatorgasket 41 and carburetor gasket 42, a connecting passage 47 can be cutinto the surface of insulator 30 which comes in contact with carburetorgasket 42, as shown in FIG. 14; or a connecting passage 51 can be cutinto the surface of the insulator 30 which comes in contact withinsulator gasket 41, as shown in FIG. 17.

The connecting passages may also be cut into the surface of cylinder 2where it is attached to the insulator 30, or into the surface ofcarburetor 12 where it is attached to the insulator 30.

When a preceding air layer-type scavenging two-stroke engine configuredin this way operates, the combustion pressure in combustion chamber 25forces piston 4 downward and exhaust port 13a opens. The combustiongases (i.e., the exhaust gas) in the combustion chamber 25 exit throughexhaust port 13a into exhaust pipe 13. They pass through the muffler(not pictured) and are released into the atmosphere.

As piston 4 goes further down, scavenging ports 9a to its left and rightopen, and the air stored in scavenging ports 109e and elsewhere flowsinto combustion chamber 25, pushing the combustion gases toward exhaustport 13a.

Next, the fuel-air mixture stored in crank chamber 5a flows intocombustion chamber 25 through scavenging ports 9a by way of scavengerpassage outlets 109b and scavenger passages 109d and 109e.

When piston 4 is at bottom dead center, exhaust port 13a and scavengingports 9a open. The supplying of air and fuel-air mixture to combustionchamber 25 is now complete or on the verge of completion. When piston 4rises from bottom dead center, it causes scavenging port 9a to close,creating a closed space in crank chamber 5a. The expansion process, thatis to say, the depressurization, now begins.

As piston 4 rises further, exhaust port 13a closes and the mixture offuel and gas in combustion chamber 25 begins to be pressurized. Whenpiston 4 goes up, the volume of crank chamber 5a increases, causing thepressure in the crankcase to decrease. When piston 4 rises stillfurther, air intake port 15a on the side of cylinder 2 opens. Thefuel-air mixture generated in carburetor 12, whose rate of flow iscontrolled by valve 14, is supplied via fuel passage 15 to crank chamber5a.

The pressure inside the crank chamber 5a drops, and the lower pressureis communicated via outlets 109b and scavenger passages 109d and 109e tobranching air passages 10a on the left and right. Reed-type non-returnvalve 16 opens, and the air supplied to air supply chamber 10 via thevalve 16 flows into crank chamber 5a.

The various pairs of passages which run from the scavenging ports 9a tocrank chamber 5a, namely, branching scavenger passages 109e andscavenger passages 109d, together form two long scavenger passages. Theair supplied to the scavenger passages must fill their entire lengthbefore it is admitted into crank chamber 5a.

When piston 4 reaches the vicinity of top dead center, spark plug 8discharges a spark in combustion chamber 25. This ignites thepressurized fuel-air mixture and combustion occurs. The pressuregenerated by this combustion pushes piston 4 down, which generatesrotary torque in crankshaft 6.

When piston 4 goes down and exhaust port 13a opens, the combustion gasesin combustion chamber 25 flow through exhaust port 13a into exhaust pipe13. They are exhausted to the exterior through the muffler (notpictured).

When piston 4 begins to drop, the gases in crank chamber 5a arepressurized by its reverse side. When piston 4 drops further, theoutlets of scavenging ports 9a on either side of it open. The fuel-airmixture supplied to crank chamber 5a as described above is sucked intocombustion chamber 25 from scavenging ports 9a via outlets 109b andscavenger passages 109d and 109e. The combustion gases (i.e., theexhaust gas) in the combustion chamber 25 are pushed out through exhaustport 13a and the scavenging operation commences.

When the engine is idling, the negative pressure in scavenger passages10b, air supply chamber 10 and branching air passages 10a becomesgreater than in fuel passage 15.

In the fourth embodiment, the air passage 10a and fuel-mixture passage15 are connected either by small-diameter connecting passages 43 and 44in insulator gasket 41 and carburetor gasket 42 or by slits 47 and 51,which are cut into insulator 30. When the engine is idling, then, thefuel-air mixture in fuel passage 15 goes through either small-diameterconnecting passages 43 and 44 or slits 47 and 51 into air passage 10band air supply chamber 10.

Thus when there is too much air in air passage 10b during suddenacceleration, fuel-air mixture can be added to the air flowing throughair passage 10b. This prevents the new air being supplied to combustionchamber 25 via scavenging port 9a from causing there to be excess air inthe chamber. It thus also prevents the fuel concentration from becomingtoo thin during sudden acceleration and so improves the engine'sacceleration characteristics.

When the engine is running at a high speed, the throttle is opened more,and the pressure differential among fuel passage 15, air passage 10b andair supply chamber 10 is virtually eliminated. Thus virtually no airflows from fuel passage 15 through the small-diameter connectingpassages 43 and 44 or 47 and 51 into either air passage 10b or airsupply chamber 10. This design, then, prevents fuel-air mixture fromcontaminating the scavenging air, which insures that the requiredexhaust specifications can be maintained.

When the engine is mounted obliquely, the fuel-air mixture in fuelpassage 15 will flow downstream from the non-return valve 16. It willthus flow through the connecting passages on the side of combustionchamber 25, namely passages 44 or 51 in insulator gasket 41 of insulator30, and into air supply chamber 10. This prevents fuel from collectingdownstream from fuel passage 15 and being sucked into the cylindersuddenly when the position of the engine changes, so it eliminatesimperfect combustion due to an excess of fuel.

To prevent defective operation when the engine is mounted obliquely, asdescribed above, the following must be considered. If the connectingpassage 44 or 51 is provided between air supply chamber 10 and fuelpassage 15, which are downstream from the non-return valve 16, it isconnected to scavenger passage 109e, which is downstream from the valve16. Thus if the diameter of the connecting passage is increased, theoutput of the engine will drop; however, if the connecting passage 43 or47 is located in carburetor gasket 42 or insulator gasket 41, which areupstream from the non-return valve, the diameter of the connectingpassage can be increased without producing a drop in engine output. Thisfurther improves the acceleration characteristics during suddenacceleration.

With the fourth embodiment, then, small-diameter connecting passages 43and 44 are provided between fuel passage 15, air passage 10b and airsupply chamber 10, so that negative pressure in the air passage 10b andair supply chamber 10 will cause the fuel-air mixture in the fuelpassage 15 to flow into the air passage. This obviates the need for acomplicated control device and allows us to achieve the effect describedabove through the use of a very simple device.

If one wishes to provide the connecting passage 43 or 44 in insulatorgasket 41 or carburetor gasket 42, it will be possible to fashion aconnecting passage merely by creating a slit of the same diameter as theconnecting passage 43 or 44 in either the insulator gasket 41 or thecarburetor gasket 42. Creating a connecting passage in this way isstraightforward and does not require a large number of processes.

In FIG. 18, which shows another embodiment of the connecting passage, 63is a connecting passage consisting of a small hole which connects airpassage 10b and fuel passage 15 in insulator 30. 64 is the non-returnvalve on the connecting passage 63 which permits flow from the fuelpassage only in the direction toward air passage 10b.

The non-return valve 64 may be eliminated, if desired, and connectingpassage 63 may consist only of the small hole. Alternatively, aconnecting passage 63 and non-return valve 64 as described above may beprovided in carburetor 12 or cylinder 2 instead of in the insulator 30.

In FIGS. 19 through 22, which show the fifth preferred embodiment ofthis invention, 1 is the engine, 2 is the cylinder of the engine and 5is the crankcase. The cylinder 2 and crankcase 5 are fastened togetherat surfaces 04 and 05 by a number of bolts 110, with gasket 311 placedbetween them. 13a is the exhaust port.

9a is the scavenging port, which opens into the side of the aforesaidcylinder 2. 209a is the scavenging passage in cylinder 2, which isconnected to the aforesaid scavenging port 9a. 209c is the outlet of thescavenging passage which opens into crank chamber 5a in the aforesaidcrankcase 5. 209b is the scavenging passage in the aforesaid crankcase5. With the help of surface 511 of guide 501, which will be discussedshortly, it forms a smoothly curved passage inside the crankcase 5.Scavenging passage 209a in the aforesaid cylinder 2 and scavengingpassage inlet 209are connected at the aforesaid surfaces 04 and 05 wherethe passages in crankcase 5 and cylinder 2 meet.

1001 is the air passage for preceding air. It is connected partway alongthe length of the aforesaid scavenging passage 209a. Preceding air fromthe air cleaner (not pictured) is supplied to scavenging port 9a throughthe air passage and scavenging passage 209a.

With the exception of guide 501, the configuration just described isidentical to the prior art design shown in FIG. 39.

501 is a guide. It is inserted into the aforesaid crankcase 5 fromsurface 04 to provide surface 511, which is smoothly connected toscavenging passage 209b in the crankcase.

As can be seen in FIGS. 21 and 22, the guide 501 has a cylindricalprotrusion 531 on its upper surface. Two teeth, 541 and 551, protrude onits sides. As can be seen in FIG. 20, when the teeth 541 and 551 on theguide 501 engage in depressions 151 and 141 in crankcase 5, the guide isfixed to the crankcase. If one set of teeth and depressions, for exampletooth 541 and depression 151, is engaged more loosely than the other, inthis case tooth 551 and depression 141, guide 501 can easily beinstalled.

Projection 531 on the guide 501 engages in the hole (not pictured) inthe aforesaid gasket 311. Thus when gasket 311 is installed on top ofguide 501, the correct placement of the gasket 311 can be checked byverifying the position of projection 531. Also, the fact that the guide501 has been installed can be confirmed by looking at gasket 311, sothere is no chance of forgetting to install the guide 501.

There is a depression 521 on the upper surface of the aforesaid guide501. When fuel from the fuel-air mixture gets into the slight gap wheregasket 311 separates the crankcase from the cylinder, the fuel will flowdownward through the depression 521.

In a two-stroke engine with a scavenging passage configured in this way,the fuel-air mixture from crank chamber 5a in crankcase 5 is conductedinto scavenging passage 209b, a portion of which consists of thesmoothly curved surface 511 of guide 501. It flows through the smoothlycurved scavenging passage 209b and is supplied to scavenging port 9a.

Because scavenging passage 209b is a smoothly curved channel without anyright angles, the fuel-air mixture flows through it smoothly and rapidlywithout any flow loss such as a decrease in flow velocity as it issupplied to scavenging port 9a.

With the fifth preferred embodiment, the aforesaid guide 501 isinstalled in crankcase 5 in such a way that it can be attached orremoved from surface 04 of cylinder 2 in the axial direction 50 of thecylinder. This obviates the need for the tooth 161 between the upperwall of scavenging passage 209b and the surface 04 to which thecrankcase is attached, as was required in the prior art. The guide 501performs the same function as the aforesaid tooth 161. Thus whencrankcase 5 is cast, even if a single die is used to form scavengingpassage 209b inside the crankcase 5, the die can easily be removed inthe axial direction 50 of the cylinder. Because the scavenging passagecan be formed using a single die, there is no possibility that one ofseveral dies will slip out of position, as sometimes happened with priorart techniques, and ruin the casting.

Also, with the fifth preferred embodiment, if the fuel in the fuel-airmixture flowing through scavenging passages 209a and 209b, which areconnected where they meet between surfaces 04 and 05 of crankcase 5 andcylinder 2, seeps into the gap between surfaces 04 and 05 where gasket311 is inserted, the fuel will flow downward through the depression 521formed in guide 501. Further, creating the aforesaid depression 521reduces the surface area of the aforesaid gap, so less fuel will seepinto the gap. Thus even if the engine is mounted obliquely, the fuelcannot return to scavenging passages 209a and 209b. This prevents theimperfect combustion which would result if fuel could flow back intoscavenging passages 209a and 209b.

In addition to the aforesaid embodiments, the following twomodifications are also included in the scope of this invention.

In the first modification, the aforesaid guide 501 consists of a flatpiece which is formed integrally to gasket 311 from a resin. The piececorresponding to the guide is formed by molding a deep-drawing sheet.This surface 511, which connects smoothly with scavenging passage 209bin the aforesaid crankcase 5, and it also forms depression 521 on thereverse side of the surface.

With this modification, guide 501 and gasket 311 are one piece, and deepdrawing allows channel surface 511 and depression 521 to be formed onthe front and reverse sides of the sheet at the same time. This reducesthe parts count and the number of assembly processes.

In the second modification, the aforesaid guide 501 is given a differentcolor than the rest of the crankcase assembly. If the guide is adifferent color, its presence or absence will be all the more evident,so it will be impossible to forget to install it. By using a differentcolor for each type of machine, we can simplify our parts control.

FIG. 27 shows an air-layer-type scavenging two-stroke engine in whichthe sixth and seventh preferred embodiments of this invention have beenimplemented. In this figure, 2 is the cylinder; 421 is the interior wallof the cylinder; 4 is the piston; 6 is the crankshaft; 6a is the crankweb, a constituent of the crankshaft 6; 5 is the crankcase; 3 is theconnecting rod which links piston 4 to crankshaft 6; 7 is the cylinderhead; 8 is the spark plug; 11 is the air cleaner; and 12 is thecarburetor.

25 is the combustion chamber; 5a is the crank chamber inside crankcase5; and 15 is the fuel passage which connects the aforesaid carburetor 12to crank chamber 5a. 13a is the exhaust port on the side of cylinder 2.It is connected to the exhaust pipe by exhaust passage 411.

9a are the two scavenging ports, which face each other on cylinder 2 tothe right and left of exhaust port 13a at virtually a right angle withrespect to the exhaust port. The scavenging ports 9a communicate withthe aforesaid crank chamber 5a via branching scavenging passages 109e,which are angled obliquely with respect to cylinder 2; arc-shapedscavenging passages 109d, which are formed inside the walls on eitherside of crankcase 5; and outlets 109b.

10 is the air supply chamber formed on the side of cylinder 2. Itsupstream side is connected to air passage 10b in insulator 30; itsdownstream side is connected to branching air passages 10a. Thebranching air passages 10a connect to the two branching scavengingpassages 109e.

Non-return valve 16 on the outlet of the air supply chamber 10, whichgoes to branching air passages 10a on the right and left, permits air toflow only in the direction of the air passages.

The aforesaid insulator 30 thermally isolates the air intake system fromthe engine body. It is bolted to the side of cylinder 2. The aforesaidair passage 10b is in the upper portion of the insulator 30 and fuelpassage 15 is in the lower portion.

The upstream side of the fuel passage 15 is connected to valve 14 oncarburetor 12, which controls the flow rate of the fuel-air mixture. Thedownstream side is connected to the inside of the cylinder (i.e., tocombustion chamber 25) via air intake port 15a.

110 is an air passage integral to carburetor 12 which connects aircleaner 11 to insulator 30. On the air passage 110 is an air controlvalve 20, which changes the diameter of the passage. The air controlvalve 20 is interlocked with mixture control valve 14 on carburetor 12.

This invention concerns an improvement in the configuration of thescavenging passages in the air layer-type scavenging two-stroke enginediscussed above.

FIGS. 23 and 25 show the sixth preferred embodiment of this invention. 2is the cylinder; 25 is the combustion chamber inside the cylinder; 43 1is the space for the aforesaid spark plug 8; 15 is the fuel passage; 15ais the air intake port which is the outlet of the fuel passage 15 intocombustion chamber 25; 13a is the exhaust port; and 411 is the exhaustpassage which connects the chamber to the exhaust port 13a. 421 is theinterior wall of the cylinder.

9a is one of the scavenging ports. It is actually the outlet of one ofthe aforesaid scavenging passages 109d into combustion chamber 25. As inFIG. 27, there are two such passages and outlets.

As is shown in FIG. 25, upper walls 9and 9b of the aforesaid scavengingpassages 109d, which connect to the aforesaid scavenging ports 9a, andtheir blow-up angle α, the angle they form with respect to a passageperpendicular to the axis 50 of the cylinder, or in other words withrespect to horizontal passage 45, vary along the periphery of thecylinder.

The sixth preferred embodiment is shown in FIG. 23. If we call theblow-up angle of the aforesaid scavenging passage 109d, which can beseen in FIG. 25, in a location nearer the aforesaid exhaust port α1 (Bin FIG. 25) and that in a location nearer the aforesaid intake port α2(A in FIG. 25), then α1<α2. The blow-up angle α varies continuously froma location nearer intake port 15a (α2) to one nearer exhaust port 13(α1).

In the seventh embodiment of this invention, which is pictured in FIGS.24, 26 and 27, surfaces 9and 9b of the upper wall of the aforesaidscavenging passage 109d are formed so that they vary in step fashionfrom blow-up angle α2 along a given length a on the side of the passagenearer intake port 15a to blow-up angle α1 along a given length b on theside of the passage nearer exhaust port 13, with the angular differencemediated by step 441.

In this case there is a fixed blow-up angle α2 along the entire lengthof the aforesaid given length a, and a different fixed blow-up angle α1along the entire length of the aforesaid given length b. It would alsobe permissible to have two or more steps like the aforesaid 441 so thatthe blow-up angle would vary in three or more stages.

When an air layer-type scavenging two-stroke engine configured in thisway operates, the combustion pressure in combustion chamber 25 forcespiston 4 downward and opens exhaust port 13. The combustion gases (i.e.,the exhaust gas) in the combustion chamber 25 flow through exhaust port13 and exhaust passage 411 to the exhaust pipe, and are released intothe atmosphere through the muffler (not pictured).

When piston 4 goes down further, the scavenging ports 9a to its left andright open. The air which has accumulated in branching scavengingpassages 109e flows into combustion chamber 25, pushing the combustiongases toward exhaust port 13.

Next, the fuel-air mixture stored in crank chamber 5 a flows intocombustion chamber 25 via outlets 109b of the scavenging passages,scavenging passages 109d and branching scavenging passages 109e.

When piston 4 reaches bottom dead center, exhaust port 13 and scavengingports 9a open, and the supply of air and fuel-air mixture to combustionchamber 25 is completed or virtually completed. Scavenging ports 9a areclosed by the action of the piston 4, and the interior of crank chamber5a becomes a closed space. As the space begins to expand, its pressurebegins to drop.

When piston 4 rises further, exhaust port 13 closes, and the fuel-airmixture in combustion chamber 25 begins to be pressurized. As piston 4rises, the volume inside crank chamber 5a increases, which furtherreduces the pressure in the crank chamber 5a. When piston 4 risesfurther, air intake port 15a on the side of cylinder 2 opens, and thefuel-air mixture generated in carburetor 12, and whose flow rate iscontrolled by valve 14, is supplied to crank chamber 5a through fuelpassage 15.

The drop in pressure inside the aforesaid crank chamber 5a iscommunicated via outlets 109b, scavenging passages 109d and branchingscavenging passages 109e to branching air passages 10a on the left andright. Non-return valve 16 opens, and the air supplied to air supplychamber 10 fills scavenging passages 109d.

When piston 4 reaches the vicinity of top dead center, spark plug 8discharges a spark in combustion chamber 25. This ignites thepressurized fuel-air mixture and combustion occurs. The pressuregenerated by this combustion pushes piston 4 down, which generatesrotary torque in crankshaft 6.

When piston 4 goes down and exhaust port 13 opens, the combustion gasesin combustion chamber 25 flow through exhaust port 13 into the exhaustpipe. They are exhausted to the exterior through the muffler (notpictured).

In the aforesaid scavenging operation, because the blow-up angle α2 ofupper walls 9and 9b of the aforesaid scavenging passage 109d is greaterat a location nearer intake port 15a than blow-up angle α1 at a locationnearer exhaust port 13a, the fuel mixture which enters the chamber fromthe location nearer the exhaust port 13a will flow along the top of thepiston at a high speed without being dispersed. This will prevent itfrom getting caught in the exhaust gas stream and so reduce the quantityof fuel lost through exhaust port 13a.

The fuel-air mixture which enters the chamber from the location nearerthe aforesaid intake port 15 of the aforesaid scavenging passage 109dwill be flowing at a lower velocity than that nearer the aforesaidexhaust port 13a. It will be sent into the area around the spark plug inthe upper part of the chamber, where it will be efficiently ignited andcombusted.

Thus the sixth and seventh embodiments prevent the fuel-air mixturesupplied to combustion chamber 25 from escaping unburned through exhaustport 13a, improve the scavenging efficiency, and increase theconcentration of the fuel-air mixture which fills combustion chamber 25.

If the engine is configured as in the aforesaid seventh embodiment, thesurfaces 9and 9b of the upper wall of the aforesaid scavenging passage109d is formed so that it varies in step fashion from a large blow-upangle α2 at a location nearer intake port 15a to a smaller angle α1 at alocation nearer exhaust port 13a, with the change in angles occurring atstep 441. When the cylinder is cast, two dies can be used with twodifferent blow-up angles α, as described above, with the angles changingat the step 441. This will make it easy to remove the work from thedies.

Also, using dies with two different blow-up angles to form scavengingpassage 109d is an easy and reliable way to control the blow-up angle.

FIGS. 28 through 31 show the eighth preferred embodiment of thisinvention. In FIG. 31, 2 is the cylinder, 4 the piston, 6 thecrankshaft, 6a the crank web of the crankshaft 6, 5 the crankcase, 3 theconnecting rod which connects piston 4 and crankshaft 6, 7 the cylinderhead, 8 the spark plug, 11 the air cleaner and 12 the carburetor. 25 isthe combustion chamber. 5a is the crank chamber formed inside crankcase5. 15 is the passage for the fuel-air mixture which connects theaforesaid carburetor 12 and crank chamber 5a. 13a is the exhaust port onthe side of cylinder 2. It is connected to exhaust passage 411.

9a are the two scavenging ports which face each other on cylinder 2 tothe right and left of exhaust port 13a at virtually a right angle withrespect to the exhaust port. As can be seen in FIG. 30, the scavengingports 9a communicate with the aforesaid crank chamber 5a via branchingscavenging passages 109e, which are angled obliquely with respect tocylinder 2; scavenging passages 109f, which are at the points where thevarious scavenging passages meet; arc-shaped scavenging passages 109d,which are formed inside the walls on either side of crankcase 5; andoutlets 109b.

The end surfaces of the aforesaid outlets 109b and the end surfaces ofcrank webs 6a approximate each other in the direction of crankshaft 60,leaving only a microscopic gap, so that the ends of the outlets can beopened and closed by the revolution of crank webs 6a of crankshaft 6.

10 is the air supply chamber formed on the side of cylinder 2. Itsupstream side is connected to air passage 10b in insulator 30, whichwill be discussed shortly; its downstream side is connected to branchingair passages 10a. As can be seen in FIG. 30, the branching air passages10a connect to scavenging passages 109f and 109e.

Non-return valve 16 on the outlet of the air supply chamber 10, whichgoes to branching air passages 10a on the right and left, permits air toflow only in the direction of the air passages.

As is shown in FIG. 30, the aforesaid branching air passages 10a andscavenging passages 109e are formed virtually symmetrically with respectto the axis 50 of the cylinder by walls 109h and 109i, which extendoutward from the sides of the cylinder 2 and are integral to it. Thewalls 109h and 109i are parallel to each other. The single die whichforms them can be removed by pulling it sideward away from the cylinder.

30 is an insulator to thermally isolate the engine body from the airintake system. The insulator 30 is bolted to the side of cylinder 2. Theaforesaid air passage 10b is in the upper portion of the insulator 30,and fuel passage 15 is in its lower portion. The upstream side of thefuel passage 15 is connected to carburetor 12, as described above. Thedownstream side is connected to the interior of the cylinder (combustionchamber 25) via air inlet port 15a.

In FIGS. 28 and 29, the aforesaid crankcase 5 consists of front portion05a and rear portion 05b. The two portions of the crankcase are dividedat surface 512, a surface at a right angle with respect to crankshaft 60and cylinder axis 50, which is also the axis of the crankshaft. Aftercrankshaft 6 and main shaft bearings 522 are installed between them, thefront portion 05a and rear portion 05b are fastened to each other by anumber of bolts 542.

533 is the flat surface on the top of the aforesaid crankcase 5 to whichthe cylinder will be mounted. The undersurface 532 of cylinder 2 isbrought up against the surface 533 and the cylinder is fastened to theaforesaid crankcase 5 by a number of bolts 552.

Inside the aforesaid front portion 05a and rear portion 05b are twosymmetrical scavenging passages 109d and their outlets 109b. The twopassages meet where they go through common surface 512. The upper endsof the scavenging passages 109d come out through the surface 533, so theupper portions of the dies which form them can be removed by pullingthem away from the surface 533.

Inside the aforesaid cylinder 2, scavenging passages 109f, which connectwith scavenging passages 109d in the aforesaid front portion 05a andrear portion 05b of the crankcase; branching scavenging passages 109e,which connect with the scavenging passages 109f; and branching airpassages 10a, which connect with the branching scavenging passages 109e,are symmetric with respect to surface 512 of the aforesaid crankcase 5.

When an air layer-type scavenging two-stroke engine configured in thisway operates, piston 4 goes down and the scavenging ports 9a to its leftand right open. The air which has accumulated in branching scavengingpassages 109e by flowing through the aforesaid air passage 10b,non-return valve 16 and air supply chamber 10 flows through scavengingports 9a into combustion chamber 25 and pushes the combustion gasestoward exhaust port 13 a.

Next, the fuel-air mixture stored in crank chamber 5a flows throughscavenging ports 9a into combustion chamber 25 via scavenging passageoutlets 109b, scavenging passages 109d and 109f and branching scavengingpassages 109e.

When piston 4 rises from bottom dead center, it closes scavenging ports9a, and the interior of crank chamber 5a becomes a closed space. As thespace begins to expand, its pressure begins to drop.

When piston 4 rises further, exhaust port 13a closes, and the fuel-airmixture in combustion chamber 25 begins to be pressurized. As piston 4rises, the volume inside crank chamber 5a increases, which furtherreduces the pressure in the crankcase. When piston 4 rises further, airintake port 15a opens, and the fuel-air mixture is supplied to crankchamber 5a through fuel passage 15.

When an air layer-type scavenging two-stroke engine configured asdescribed above is manufactured, scavenging passages 109d are castinside front portion 05a and rear portion 05b of the crankcase, and therequired machining processes are performed. Then piston 4, crankshaft 6,connecting rod 3 and main shaft bearings 522 are assembled between thetwo halves of the crankcase. The halves are joined at surface 512 andfastened together by bolts 542 to achieve a unitary crankcase 5.

In cylinder 2, scavenging passages 109f, which connect with passages109d in the aforesaid crankcase 5; branching scavenging passages 109e,which connect with the passages 109f; and branching air passages 10a,which connect with the branching passages 109e, are cast in such a waythat they are symmetric with respect to surface 512 of the aforesaidcrankcase 5, and the required machining processes are performed.

Piston 4, connecting rod 3 and other necessary components are assembledin the aforesaid cylinder 2, and its undersurface 532 is brought upagainst surface 533 of the aforesaid crankcase 5. The cylinder is thenfastened to the crankcase by bolts 552.

With the eighth embodiment, then, crankcase 5 is divided at surface 512,a surface at a right angle with respect to crankshaft 60 and cylinderaxis 50, which is also the axis of the crankshaft, into a front portion05 a and a rear portion 05b, both of which have scavenging passages 109drunning inside them. The front and rear portions 05a and 05b arefastened together by bolts 542. Cylinder 2, which contains scavengingpassages 109f, branching scavenging passages 109e and branching airpassages 10a, is fastened to surface 533 on the top of the aforesaidcrankcase 5 by bolts 552. Scavenging passages 109f on the bottom of thecylinder communicate with scavenging passages 109d in crankcase 5,forming two long scavenging passages which run through both crankcase 5and cylinder 2. The crankcase 5 and cylinder 2 thus assume a compactshape with no projections, and their scavenging passages are smoothpassages with no sharp angles.

The engine is divided into front and rear portions 05a and 05b, withscavenging passages 109d running through both portions, and the twoportions are cast separately. Thus the work can be removed from the diesat the aforesaid surface 512 where the two portions will be joined or atsurface 533, the surface perpendicular to surface 512 where the cylinderis mounted. This simplifies the shapes of the dies and the removal ofthe work and reduces the number of dies needed.

With the eighth embodiment, scavenging passages 109d and 109f, branchingscavenging passages 109e and branching air passages 10a are allsymmetric with respect to surface 512 of the aforesaid crankcase alongtheir entire length from outlets 109b into crank chamber 5a toscavenging ports 9a in cylinder 2. A common die can therefore be used tocast the two respective scavenging passages and branching air passagesin the front and rear portions of the crankcase, so fewer dies areneeded. The shape described above makes the passages of the twosymmetric scavenging passages exactly the same size. Cylinder 2 willtherefore be filled uniformly along its circumference with scavengingair and fuel-air mixture. And because the aforesaid two branching airpassages 10a also have identically-shaped passages, cylinder 2 will bescavenged uniformly along its circumference.

Walls 109h and 109i of the aforesaid scavenging and branching airpassages can be formed integrally to cylinder 2 and virtually parallelwith each other. Because this design allows the cylinder to be castusing a single sliding die, it simplifies the configuration of the dieand reduces the cost of producing it.

FIGS. 32 through 38 show the ninth preferred embodiment of thisinvention. In FIG. 32, 2 is the cylinder; 5 is the crankcase. Thecylinder 2 and crankcase 5 are fastened to each other with gasket 05between them by bolts 110 at their mounting surfaces. 4 is the piston; 3is the connecting rod; 8 is the spark plug; 13a is the exhaust port; and25 is the combustion chamber.

9a is the scavenging port, which is on the side of the aforesaidcylinder 2. 5a is the crank chamber inside the aforesaid crankcase 5. 12is the carburetor; 30 is the insulator between the carburetor 12 and theaforesaid cylinder 2. 15 is the passage for the fuel-air mixture. Itruns from the throttle passage of the aforesaid carburetor 12 throughinsulator 30 and cylinder 2 to the aforesaid crank chamber 5a. 10b isthe air passage. It runs from the air passage of the aforesaidcarburetor 12 through insulator 30 and non-return valve 16 to thescavenging passage and the aforesaid scavenging port 9. The non-returnvalve 16 is a reed valve which is opened and closed by negative pressurein the aforesaid scavenging passage.

In the two-cycle engine according to this invention, the air cleaner andthe choke valve used in it have been improved.

1001 is the air cleaner, which is configured as follows.

1011 is the air cleaner housing. It is attached to the aforesaidcarburetor 12 by bolts (not pictured). 1021 is the air cleaner cover. Itis attached to the aforesaid air cleaner housing 1011 by bolts 109.

Two air passages, passage 1061 and passage 1071, run parallel to eachother through the aforesaid air cleaner housing 1011. Air passage 1061connects to air passage 10b, which has the aforesaid non-return valve 16on it. Air passage 1071 connects to the aforesaid fuel passage 15.

1201 is a choke valve which alternately opens and closes the aforesaidair passages 1061 and 1071.

In FIGS. 32 through 38, 1041 is a rotary valve consisting of valveportions 1041a and 1041b. When rotated, it engages over the opening inthe center of the aforesaid air cleaner cover 1021. Valve portions 1041aand 1041b alternately open and close the inlets of the aforesaid airpassages 1061 and 1071. 1031 is the rotary knob which operates the valve1041. It is fixed to the end of the valve 1041. There is a choke hole1501 in valve portion 1041b. On the side of valve portions 1041a and1041b which is away from the engine extensions 1511 and 1521 are formedintegrally to the valve portions.

The end of the aforesaid valve 1041 consists of flat surface 1101, whichcovers or uncovers the inlets of the aforesaid air passages 1061 and1071. Flat surface 1101 of the valve 1041, as can be seen in FIG. 32, isshaped in such a way as to completely obstruct the inlets of theaforesaid air passages 1061 and 1071 when the valve 1041 is rotated.

1501 is an O-ring which is placed on the circumference of the valve stemof the aforesaid valve 1041. It goes between the inner surface 1131 ofthe aforesaid air cleaner cover 1021 and step portion 1221 of the valvestem of the aforesaid valve 1041 and exerts pressure in the axialdirection of the valve, forming a liquid seal for the interior of aircleaner 1001. Its elastic force presses flat surface 1101 of theaforesaid valve 1041 against the inlets of the aforesaid air passages1061 and 1071.

As FIG. 33 shows, when rotary knob 1031, which is integral to theaforesaid valve 1041, is turned approximately 90° clockwise fromstarting position, it is set in normal operating position. 1231 is astop which projects from the exterior surface of the aforesaid aircleaner cover 1021. As can be seen in FIGS. 33 and 34, when theaforesaid rotary knob 1031 is being rotated from the aforesaid startingposition to normal operating position, its end goes over the aforesaidprojection 1231 against the force of the aforesaid O-ring 1051.Extensions 1511 and 1521 on valve 1041 come up against and are stoppedby stops 1531 and 1541, as shown in FIG. 37. The elastic force of theaforesaid O-ring 1051 returns the valve to its previous position, and itis held on the portion of the aforesaid air cleaner cover 1021 whichdoes not have a projection 1231 on it.

As can be seen in FIGS. 37 and 38, there is a projection 1111 with atapered surface 1121 on the inside of the aforesaid air cleaner cover1021. When the aforesaid rotary knob 1031 is in starting position, asshown in FIG. 33, its end rides up on tapered surface 1121 of theprojection 1111, and flat surface 1101 of the aforesaid valve 1041pushes against the inlet of the aforesaid air passage 1061, closing itcompletely.

In an air cleaner in a two-stroke engine configured as described above,when the engine is going to be started up, the rotary knob 1031 of chokevalve 1201 is set in starting position (choke position), as shown inFIG. 33. When rotary valve 1041, which is fixed to the knob 1031, isrotated, the air inlet of carburetor 12 and the aforesaid air passage1071, which is connected to the aforesaid fuel passage 15, is fullyclosed (although the small choke hole 1501 remains open). The aforesaidair passage 1061, which is connected to the air passage 10b whichsupplies preceding air, is also fully closed, and the engine is startedup.

When this choke valve 1201 is operated, the air filtered by cleanerelement 1081 of air cleaner 1001 flows into air passage 1071 throughchoke hole 1501 and is supplied from the passage 1071 to the main nozzleof carburetor 12, which is connected to passage 1071. In the carburetor12, a fuel-air mixture is generated by atomizing the fuel in the air.The fuel-air mixture is supplied from fuel passage 15 through crankchamber 5a, the scavenging passage and scavenging port 9a, intocombustion chamber 25, where it is ignited and combusted, thus startingthe engine.

When the engine is started up, air passage 1061 of air cleaner 1001 isfully closed by valve 1041 of the aforesaid choke valve 1201. Thuspreceding air cannot be supplied to the combustion chamber through theair passages 1061 and 10b. Only the fuel-air mixture generated incarburetor 12 using the air which enters the aforesaid air passage 1071through choke hole 1501 is supplied to the combustion chamber. Thuscombustion chamber 25 will be filled with a rich fuel-air mixture, andthe engine's starting characteristics will improve.

Flat surface 1101 of rotary valve 1041 in the aforesaid choke valve 1201opens and closes the inlets of the aforesaid air passages 1061 and 1071.1051 is an O-ring which is placed on the circumference of the stem ofthe aforesaid valve 1041 between the inner surface 1131 of the aforesaidair cleaner cover 1021 and step portion 1221 of the stem of theaforesaid valve 1041. It exerts pressure in the axial direction of thevalve 1041, forming a liquid seal for the interior of air cleaner 1001.Its elastic force presses flat surface 1101 of the aforesaid valve 1041against the inlets of the aforesaid air passages 1061 and 1071. Thus thechoke valve 1201 completely closes the inlet of air passage 1061 forpreceding air. This allows a rich mixture to be generated, as describedabove, and it allows a high negative pressure to be maintained.

In addition to the aforesaid O-ring 1051, a projection 1231 which servesas a stop is provided on the outer surface of the aforesaid air cleanercover 1021. When the aforesaid choke valve 1201 is rotated from startingposition to normal operating position, it switches between the aforesaidair passages 1061 and 1071. When the aforesaid rotary knob 1031 isbetween starting position and normal operating position, its end goesover the aforesaid projection 1231 against the force of the aforesaidO-ring 1051. The moderate friction improves the operating feel of thechoke valve. When the choke valve 1031 is released, the elastic force ofthe aforesaid O-ring 1051 and the force of the aforesaid protruding stop1231 automatically hold the choke valve 1201 in place on the flatportion of the outer surface of air cleaner 1021 in such a way that itcannot go back.

There is a projection 1111 with a tapered surface 1121 on the inside ofthe aforesaid air cleaner cover 1021. As can be seen in FIG. 33, whenthe aforesaid choke valve 1201 is rotated toward starting position, theend of rotary knob 1031 will ride up on tapered surface 1121 of theprojection 1111 when the knob reaches starting position. The flatsurface 1101 of the aforesaid valve 1041 will then press against theinlet of the aforesaid air passage 1061.

This improves the seal formed by flat surface 1101 of the valve 1041 sothat the aforesaid air passage 1061 can be completely closed off.

A rotary choke valve 1201 configured as described above is not limitedin its application to use as a choke valve for an air cleaner asdescribed above. It can be used for a wide range of applications whichrequire a valve to switch between two fluid passages when knob 1031 isturned to rotate valve 1041.

1. A two-stroke cycle engine, comprising: a scavenger passage whichconnects a scavenging port on the side of the cylinder to the crankchamber inside the crankcase, and goes through the mounting surfacewhere the cylinder and crankcase are attached to each other; and aremovable guide with a surface forming a curved smooth channel which isattachable to said scavenger passage in the crankcase from the mountingsurface, and forms a portion of said scavenger passage with the curvedchannel.
 2. A two-stroke cycle engine according to claim 1, wherein saidremovable guide comprises a positioning tooth which engages with thehole in the gasket for the mounting surface where the cylinder andcrankcase are attached to each other.
 3. A two-stroke cycle engineaccording to claim 1, wherein said removable guide is fixed to thecrankcase when a tooth engages in an indentation in the crankcase.
 4. Atwo-stroke cycle engine according to claim 1, wherein said removableguide has a depression in the mounting surface where the cylinder andcrankcase are attached to each other.
 5. A two-stroke cycle engineaccording to claim 1, wherein said removable guide is painted on.
 6. Atwo-stroke cycle engine, comprising: an exhaust port on the sidewall ofthe cylinder, which opens into the cylinder; a scavenging port on thesidewall of the cylinder positioned a slight distance apart in thecircumferential direction from said exhaust port, which also opens intothe cylinder; an intake port, which opens to supply fuel-air mixture tothe crankcase according to the action of the piston; and a scavengerpassage, which connects the crankcase and said scavenging port; whereina blow-up angle (α) of said scavenger passage, which is defined by anangle between the upper wall which connects to said scavenging port anda perpendicular line to the axis of the cylinder, varies along thecircumferential direction of the cylinder, and if said blow-up angle ina location nearer said exhaust port is defined as α1) and said blow-upangle in a location nearer said intake port is defined as (α2), thenα1<α2; said blow-up angle α varying in step fashion from a locationnearer intake port (α2) to said blow-up angle nearer exhaust port (α1).